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Runway Position Update

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If performing a runway intersection takeoff and with GPS enabled (GPS update didables TOGA update) will i still need to enter the RWY REMAIN. Or will the GPS account for intersection distance?

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Another bizarre question. Try the intersection takeoff and see what happens. Use the default panel state so you don't spend a lot of time setting up a flight that might crash. Start the flight at the end of the runway and taxi to the intersection you want to try.

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If performing a runway intersection takeoff and with GPS enabled (GPS update didables TOGA update) will i still need to enter the RWY REMAIN.

 

This is the sole purpose of this feature.

 

 

 


Or will the GPS account for intersection distance?

 

It will mix in, but the IRS position error will remain, I believe.

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I just can't find any reference to add TO SHIFT or RWY REMAIN if GPS is updating.

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I just can't find any reference to add TO SHIFT or RWY REMAIN if GPS is updating.

 

Hyperfocusing again - not a big deal if you don't do it (and the lack of mention in FCOMv2 is an indicator of this), but why not get into a good habit of it for the times GPS is MELd?

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But thats not the answer i was looking for? Does GPS auto update the intersection in the FMC POS?

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More to the point, if you are flying jets, don't get into the habit of accepting intersection takeoffs. This will solve your issue and provide you with an extra element of safety. When flying, always keep your options in your back pocket for those times you will need them.

 

Remember:

 

Runway Behind You,

Altitude Above You,

Fuel You Left At The Gate,

And, You Only Have Too Much Fuel When You Are On Fire

 

Revert to your manual for the required numbers whenever you find that the GPS will not provide you with the required information. As was stated by Kyle, if your GPS fails, you have to have an alternate method of data retrival.

 

If, though, you find yourself accepting an intersection departure, plug in the departure runway, set up your initialization, obtain your "full length runway performance data", and compare that data to the available runway distance from the intersection departure point. If there is an issue with the data and a performance issue manifests itself, don't perform the intersection departure.

 

Cheers,

Jim Wilkerson


But thats not the answer i was looking for? Does GPS auto update the intersection in the FMC POS?

 

Vernon, unless something has really changed in the past couple of years in regards to GPS, GPS will not update performance data from a position update. GPS knows full runway length data and doesn't provide performance data for anything but full runway length available. The only variables I am aware of will be takeoff weight, flap setting, runway slope, and runway contamination.

 

I personally haven't seen a GPS that will recalculate runway available from a positional update with regards to a particular runway selected to be used for departure in the GPS. Almost every database for GPS provides only the runway selected "Full Length" data. I don't know of any GPS that will extrapolate remaining runway available, and provide "updated" takeoff performance numbers from a positional update.

 

Cheers,

 

Jim Wilkerson

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But thats not the answer i was looking for? Does GPS auto update the intersection in the FMC POS?

 

The earlier post kinda does.

 

Remember that we're using MIX data here. Go to the POS page to see all the individual positions being used. They're all usually different, though the GPS positions are usually pretty close. The IRS position, once aligned, is never updated until it is turned off and back on again (one of the reasons it's recommended to do so on the turn).

 

Is this a problem?

As long as you have GPS and/or DME/DME updating, no. The GPS is has the highest priority when feeding position updates, so as long as it's working just fine, then the TAKEOFF entry is relatively moot. If it were to drop, however, you're left with a compromised IRS position by however many feet was cut off of the runway using the intersection departure. That isn't much, but disasters are usually a chain of events, and by not entering the change, you're consciously operating with a bent link. We do that all the time in aviation, honestly (see MEL items), but those are calculated risks. For such a simple entry, though, I'd argue that regardless of the magnitude of risk, you might as well just do it.

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More to the point, if you are flying jets, don't get into the habit of accepting intersection takeoffs. This will solve your issue and provide you with an extra element of safety. When flying, always keep your options in your back pocket for those times you will need them.

 

Remember:

 

Runway Behind You,

Altitude Above You,

Fuel You Left At The Gate,

And, You Only Have Too Much Fuel When You Are On Fire

 

 

Excellent private pilot level advice. The next time you're taxiing out at O'Hare and you're assigned 28R at MM or EE, and you see the line of 20+ aircraft ranging from RJs to 747s waiting to take those departures, you go ahead and advise ground you'll require full length in your 737. Enjoy the penalty box... But hey, MAJOR overs! :-D

 

Intersection departures are a reality of airline ops at many large airports....

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Thanks kyle and also to the rest! But, the runway coordinate in the FMC Nav data base and EFIS map runway symbol is the landing threshold!? So departing other than the landing threshold surely this will introduce an FMC position error even with working GPS or DME/DME. Hence entering RWY REMAIN regardless....

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Thanks kyle and also to the rest! But, the runway coordinate in the FMC Nav data base and EFIS map runway symbol is the landing threshold!? So departing other than the landing threshold surely this will introduce an FMC position error even with working GPS or DME/DME. Hence entering RWY REMAIN regardless....

It isn't necessary with GPS Updating. Just remember why the TOGA position update is there at all. It's a feature that dates from before GPS. TOGA position update was meant as a way to remove any position error that might have been there before departure. Incorrect initial position entry for example. When TOGA is selected, the FMC position gets a position correction to the runway threshold plus any intersection shift you entered. If you have GPS updating installed then this is completely irrelevant. FMC position will be updated by GPS corrections throughout the flight. You also have DME position update.

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Excellent private pilot level advice. The next time you're taxiing out at O'Hare and you're assigned 28R at MM or EE, and you see the line of 20+ aircraft ranging from RJs to 747s waiting to take those departures, you go ahead and advise ground you'll require full length in your 737. Enjoy the penalty box... But hey, MAJOR overs! :-D

 

Intersection departures are a reality of airline ops at many large airports....

 

Yeah - I'm a huge proponent for safety, but I'm also very outspoken about black and white safety rules boxing people into some operations that may actually be detrimental to the operation, and in some cases, even less safe. There's no shortage of it in the industry, which is somewhat worrisome, if only for the fact that regulated safety takes the conscious safe decision-making out of the mix, which is really what we need as pilots.

 

As a recent example, I know a bunch of the flight schools in the area have policies restricting pilots from intersection takeoffs. Manasty (KHEF) is going through some growing pains (taxiway and run-up area resurfacing/widening), so when the field is in South Ops, it's 16L at B2 (unless full length is requested). That cuts a significant portion off of the runway, but it's still just about as long as 16R, which the school regularly uses. Having operated on much smaller runways (2000x40 is the smallest, I believe, and the shortened 16L is nearly double that), I took the lowly Skychicken out on 16L at B2 and got out of there.

 

It saved me time, saved the controller time, and left me with less exposure to departing/arriving traffic (what people seemingly fail to see when they unnecessarily require full length in certain operations is that being on the runway is an exposure to the risk of being hit by arriving or departing aircraft - back taxiing greatly increases this exposure at both controlled and uncontrolled fields).

 

Thanks kyle and also to the rest! But, the runway coordinate in the FMC Nav data base and EFIS map runway symbol is the landing threshold!? So departing other than the landing threshold surely this will introduce an FMC position error even with working GPS or DME/DME. Hence entering RWY REMAIN regardless....

 

Earlier, my points were more related to getting into the habit of doing it just to avoid any time where you set yourself up for a bad position update if GPS were MELd. The reason for this, as mentioned, is the nature of the mix. IRS position will never change, but the position update will help the FMC use the IRS position and correct what it provides when coming up with an FMC position "solution" (regardless of GPS availability). GPS being available simply takes a higher priority (because it is considered more accurate by itself), but to increase the accuracy of the position and provide verification/redundancy, the GPS position is mixed with the other positions (individual IRSs and DME/DME). The fact that GPS has a higher priority in the mix means that, as long as GPS is available, the position error introduced in not using the RWY REMAIN function is negligible.

 

To put it in perspective, if the runway were shortened by 1500 feet, and your position was based solely off of the position update taken on hitting TOGA, you'd still theoretically be able to maintain RNP 0.3 (assuming no position drift over time). Given that this position update is actually mixed in with the provided IRS positions (remember, it doesn't overwrite them), along with DME/DME, the actual error is much less than 1500 feet as the IRS positions and DME/DME positions will show a position closer to "reality." Adding GPS back in, particularly given its higher priority in the mix, means that the position error from the RWY REMAIN is nearly flat out ignored (since there are better sources of information available: IRS, DME/DME, and GPS all somewhat disagreeing with the TOGA value).

 

So, again, it's something that's nice to be in the habit of just in case the GPS is out of service, but it's not something to be incredibly concerned about.

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Earlier, my points were more related to getting into the habit of doing it just to avoid any time where you set yourself up for a bad position update if GPS were MELd.

I'm confused now. Are you saying that the FMC records the TO shift anyway when TOGA is activated but doesn't use it if GPS update is enabled? That's the only way it could introduce the shift later in the event of a GPS problem. Yet FCOM v1 says you should use radio navaids to manually update FMC position in that case.

 

It may be a good habit to get into, but I'm not sure it actually achieves anything, even if GPS goes U/S.

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I'm confused now. Are you saying that the FMC records the TO shift anyway when TOGA is activated but doesn't use it if GPS update is enabled? That's the only way it could introduce the shift later in the event of a GPS problem. Yet FCOM v1 says you should use radio navaids to manually update FMC position in that case.

 

It may be a good habit to get into, but I'm not sure it actually achieves anything, even if GPS goes U/S.

 

Speaking from a pure data perspective, and I'm not sure of the actual intricacies here, but:

 

Say we're in an IRS-only mode (no GPS, no DME/DME):

  • You spin up both IRUs. Both are devoid of assumed position until you enter the location on POS INIT. Both IRUs will use this position, though they will both diverge naturally through the natural inertial errors. After that initial entry, no actual updates to the position are accepted on the IRUs, which means that any updates are mixed in to correct any IRU errors, and not to reset the position of the IRUs.
  • The FMC will pull the runway threshold when TO/GA is pressed and enter it into the calculated FMC position. Again, since the IRU positions are never overwritten after the initial alignment, this pulled value is simply mixed in with the IRU-provided positions. My thought is that, when TO/GA is pressed, the value is used to check against each IRU-provided position to assist in generating an initial error value (figure of merit) for each.
  • As the FMC single position is calculated from the IRUs, the FMC will use the these metrics to determine a more accurate assumed position.

Now, add in DME/DME:

  • Assuming all of the above, DME/DME updating adds more position data to the mix, in addition to the individual IRU positions.
  • This bias, in this case, is about 80:20 in favor of the DME/DME data, because it is based on fixed, ground-based navaids.
  • IRU positions remain independent, so the initial FoM is still factored into the IRU mix.

Now add in GPS:

  • Assuming all of the above, GPS updating adds even more position data to the mix, in addition to RADIO and IRU.
  • The bias with GPS available is nearly entirely GPS, from what I can tell, with the other positions simply relegated to standby.
  • IRU positions, again, remain independent, so the initial FoM is still factored into the IRU mix, but has no effect as long as GPS is active.

 

 

So...from what I understand, it's "used" in the sense that the value is taken and stored until the IRUs are reset, but it's not of any real importance as long as GPS is in use.

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So...from what I understand, it's "used" in the sense that the value is taken and stored until the IRUs are reset, but it's not of any real importance as long as GPS is in use.

 

Yet the FCOM states that TO/GA position update is inhibited if GPS update is enabled. It can't be used in the FMC position calculation if it has been inhibited. As you say it has nothing to do with IRS positions, it's the FMC position which would be shifted. I can't see why such a shift would be retained in case of a failure of GPS. IRS drift could easily make such an update inaccurate and the FCOM procedure for GPS FAIL says you should establish a position fix using radio aids to enter into the FMC POSN page in such an event. That would overwrite any TOGA shift correction anyway.

 

Maybe a 737 pilot can shed some light on this.

 

edit

 

Entering the takeoff distance correction may well be important for V speed calculations (it will delete V speeds) but I think that's the only reason to enter it if you have GPS updates enabled.

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Yet the FCOM states that TO/GA position update is inhibited if GPS update is enabled. It can't be used in the FMC position calculation if it has been inhibited. As you say it has nothing to do with IRS positions, it's the FMC position which would be shifted. I can't see why such a shift would be retained in case of a failure of GPS. IRS drift could easily make such an update inaccurate and the FCOM procedure for GPS FAIL says you should establish a position fix using radio aids to enter into the FMC POSN page in such an event. That would overwrite any TOGA shift correction anyway.

 

You're correct. I missed the "TO/GA position update inhibited if GPS UPDATE is ON."

 

While we're getting into such minutia at this point that it really doesn't matter, I wanted to address the stat point of view, as the value is not entirely insignificant:

 

An initial figure of merit calculated from comparing the IRS-computed positions versus a known location is not rendered inaccurate by time. As an example, if one IRU somehow drifted more than the other between spin up and departure (as caught by the position verification at TO/GA), it's not like it's going to suddenly reverse this trend and get more accurate over time. Rather, the initial determination to not trust (or trust less) the IRU with higher error would likely continue through the flight. Again, when it comes to position, here, it's not like the runway position is taken, and the position is somehow factored into the flight for its duration, or is somehow entered as a single data point. Rather, the positions from the IRUs are compared to a known point at a point in time to determine their reliability in determining a location against a known value. There's nothing to "overwrite" here.

 

Given the inhibiting behavior with GPS enabled, it seems that the GPS solution is considered a relatively "known" location while it is available. As such, a continuous figure of merit can be calculated using the GPS as a continuous comparative source, vice a single known position right at takeoff.

 

In any case - again - this isn't an overwrite anywhere. It's a comparative evaluation. This is related to the concept of RNP/ANP: how well can this machine determine where it is in the world? If its own determination of its position yields relatively little variance between the systems (IRS, DME, GPS, etc), then ANP is low. Older versions of IRU/IRU/DME only simply used the position update as a corrective measure to interpret what the IRUs were providing for the rest of the flight. DME, of course, augmented this interpretation by adding more known position points through the rest of the flight.

 

At least that's how I understand it...

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"OH, but that could never happen to ME!"  ;-D

 

I am one of many excellent pilots that will use my excellent and widely accepted poor judgment and accept an intersection departure and save time. After all, nothing ever happens and we save time! We don't risk a thing! Forget about the how many meters of runway that will be to our back on departure and of no use to us should we experience an encounter that would have made those however many meters of now useless runway useful. ;-(

 

Forget about the passengers in the back who are counting on us to make good decisions up here to make sure that they are as safe as possible. After all, we are going to save time. :-D

 

The box says that I am required to have 3,649' of runway for our conditions for this departure. My having 4,370' of runway to depart from when I can have 7,600' of runway is just fine. And, we will save time. :-D

 

Now, that's what I call using Great Pilotage Decision Making. Whether you are a student pilot or a multi-thousands of hours pro jetjock. :-D

 

Aviation is dangerous just on it's own merits. Made even more so by so many who have come to believe it okay to do what everyone else does. In spite of the tools we have available to minimize our exposure to the inherent dangers involved with aviation. :-I

 

And, made even more more so when I take to the skies! That is why it is mandatory that regulatory entities issue NOTAMs prior to any flight I make. :-/

 

Excellent private pilot level advice. The next time you're taxiing out at O'Hare and you're assigned 28R at MM or EE, and you see the line of 20+ aircraft ranging from RJs to 747s waiting to take those departures, you go ahead and advise ground you'll require full length in your 737. Enjoy the penalty box... But hey, MAJOR overs! :-D

Intersection departures are a reality of airline ops at many large airports....

 

 

I have never had ATC get upset with me because I didn't accept an intersection departure. Maybe some day they will. They would get mad at me for contacting the tower when I was told to monitor the tower frequency. Especially at an airport in Denver.

 

No thanks, I'll wait my turn. No penalty box is a good place to find yourself in. But I'll gladly accept the penalty box for the lost time over the penalty box when your poor cockpit judgment has to be explained as to why you went off the end of the runway.

 

But, going off the end of the runway following an intersection departure never happens.....So, "what difference at this point does it make"?

 

Maybe me just being lucky. In the hundreds of commerial airline flights that I have been subjected to, not once have the flight crews performed an intersection departure. We always departed from the proper departure point of the runway....using the full length of the runway. Thankfully, they weren't in a big hurry. I know I wasn't.

 

 

Yeah - I'm a huge proponent for safety, but I'm also very outspoken about black and white safety rules boxing people into some operations that may actually be detrimental to the operation, and in some cases, even less safe. There's no shortage of it in the industry, which is somewhat worrisome, if only for the fact that regulated safety takes the conscious safe decision-making out of the mix, which is really what we need as pilots.

 

As a recent example, I know a bunch of the flight schools in the area have policies restricting pilots from intersection takeoffs. Manasty (KHEF) is going through some growing pains (taxiway and run-up area resurfacing/widening), so when the field is in South Ops, it's 16L at B2 (unless full length is requested). That cuts a significant portion off of the runway, but it's still just about as long as 16R, which the school regularly uses. Having operated on much smaller runways (2000x40 is the smallest, I believe, and the shortened 16L is nearly double that), I took the lowly Skychicken out on 16L at B2 and got out of there.

 

It saved me time, saved the controller time, and left me with less exposure to departing/arriving traffic (what people seemingly fail to see when they unnecessarily require full length in certain operations is that being on the runway is an exposure to the risk of being hit by arriving or departing aircraft - back taxiing greatly increases this exposure at both controlled and uncontrolled fields).

 

"Yeah - I'm a huge proponent for safety, but...."

 

Do you ever have a gut feeling or a shiver up the spine prior to doing something wrong or when making a poor judgment call? That's your "conscious safe decision-making" gnawing at you telling you to reconsider what you are getting ready to do. Safety is compromised when you cut corners.

 

You think "outside the box" to solve problems. Thinking "outside of the box" that creates a potentially bad situation is counterproductive.

 

".......black and white safety rules....." are not black and white. They are as red as the spilled blood that brought them about in becoming rules.

 

On a hot and humid day here, I watched a Diesel 9 depart to the east from where we based our aircraft. They started their roll from the proper departure point of the runway, black smoke trailing. The aircraft rotated with just a little less than 400' remaining of a 7,600' runway. They crossed over a set of powerlines by no more than 50'. Prior to crossing the powerlines, I didn't think they were going to clear the lines and that I was going to witness something tragic. God was on their side that day.

 

They had "The Numbers", otherwise they would not of departed.

 

Now, image the situation following an intersection departure under the very same conditions, using the same runway for departure.

 

As for the flight schools, most of the instructors have less than 400 to 500 hours, and their students are following their instructors guidance. This is not a jab at instructors as everyone has a starting point somewhere. Even most experienced instructors will tell you about one or two things they did when they were "young and dumb and ...... " that they wouldn't even consider doing after they learned otherwise.

 

At a major airport here in the U.S., during non rush hour arrivals and departures for the field, a flight instructor and his student were struck by a jet and killed. They were the only aircraft waiting to cross the active runway when the jet on departure, following an engine power loss prior to V1, left the runway and hit the small single engine aircraft.

 

There was very little arrival and departure traffic out of this airport during that time.

 

So, exposure was .....?

 

For the instructor and the student, exposure was 100%. 100% during a slow period of arrivals and departures.

 

What most people don't remember in the rush of things is that our Takeoff Performance is predicated on optimum performing powerplants and braking systems. Basically, new equipment.

 

And, for those Takeoff Perfomance numbers to be of any good, even with a "Hey, we have a problem!" momentary time lapse, the aircraft has to be relatively new or kept in relatively new condition. After all, test pilots who knew just when and what was going to occur produced those numbers, flying new equipment.

 

Sitting in the cockpit, barreling down the runway at "OH TOO FAST", having a failure, recognizing the failure, and responding to the failure with remaining runway in front of the nose of the aircraft can be quite the event. Even more the event when you very rapidly see the end of the runway coming and you still are going too fast. "OH, if only I had just another 500 to 1,000' more runway!"

 

So, having said the prior, every aircraft you fly is in nearly "new" condition. No worn brakes, bad cylinders, no hydraulic problems, etc. A lot of folks wish they could say the same.

 

Even more during these times. What with economics being the issue and most commercial operations cutting back on maintenance to save money. And, its okay now, more than ever, to perform intersection departures?

 

Next commercial flight you get on, take a look into the cockpit and view the "INOP" stickers placarded around the cockpit. They are legal by regulation and maintenance can be deferred for whatever time frame the company has had approval for. But, quite the confidence building view and the nice warm and fuzzy feeling as you go barreling down the runway following an intersection departure.


When all heck breaks loose and something happens, when meeting with your local aviation authorities and explaining YOUR outstanding command decision making procedures as justification for cutting corners, remember my "private pilot level" advice about "runway behind you".

 

Cheers,

 

QUOTE:  "Don't be a show-off. Never be too proud to turn back. There are old pilots and bold pilots, but no old, bold pilots."     E. Hamilton Lee, 1949

 

Jim Wilkerson

ATP Rank: MAJOR OVERDOER

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In any case - again - this isn't an overwrite anywhere.

 

Perhaps you are right but my understanding was that the TOGA map shift is an instant position update. Assuming no GPS and no DME updating going on, the IRS continues to give the FMC a slightly wrong position. The TOGA update on the runway gives the FMC an exact position fix, in the form of an offset from the IRS position input. That offset will keep being added to the IRS position to correct it. So it isn't necessarily part of the input mixing you described, but a one off error correction to that.

 

I just tried this in the NGX, switching off GPS updating and entering an incorrect IRS initial position (I used the airport lat/long). At the threshold the ND map showed me off to the right and behind the threshold. When I selected TOGA it didn't update the position to the runway threshold. After liftoff the FMC position shifted nearer to the correct position, nearly but not quite on the runway axis. The timing of the map shift was unexpected, but it was an instant change. It could have been a DME position update.

 

I tried again with DME updates off as well and this time there was no map shift, before or after takeoff, so it must have been a DME update that changed it. It doesn't look like the TOGA position update is working in the NGX.

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Kevin that is correct, the error will be corrected after DME-DME updating is established.

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The safest plane is probably in a museum with all the fluids drained and a nice set of sprinklers ready to let loose a deluge in case of fire.

 

Seriously, there's a rational amount of safety required and judgement is required.

 

If you saved 500ft off runway, but lost 500lbs of fuel waiting for it, were you any safer? You added 2000 pounds of fuel for weather deviation enroute, but now you're to heavy to climb above the cells. Any safer? You went to a runway the wind was favoring, but you had to fly over high terrain. Were you any safer? You delayed departure 2 hours for weather. Now you're midflight and you realize you're in the middle of microsleeps because of the long sit at the airport. Hmmmm.

 

Follow the regs and SOPs. Visit AvHerald and 37000feet to see what not to do. And listen to your gut.

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Maybe me just being lucky. In the hundreds of commerial airline flights that I have been subjected to, not once have the flight crews performed an intersection departure. We always departed from the proper departure point of the runway....using the full length of the runway. Thankfully, they weren't in a big hurry. I know I wasn't.

 

They may not have performed an intersection departure, but I can near guarantee you that some form of reduced thrust -- possibly a reduction of up to 25% -- was used in almost all of those flights, which in itself erodes margins if you want the maximum amount of runway available to stop on.

 

Who knows, they may also have used optimised speeds which kept them on the runway longer than strictly necessary as well.

 

 

 

On a hot and humid day here, I watched a Diesel 9 depart to the east from where we based our aircraft. They started their roll from the proper departure point of the runway, black smoke trailing. The aircraft rotated with just a little less than 400' remaining of a 7,600' runway. They crossed over a set of powerlines by no more than 50'. Prior to crossing the powerlines, I didn't think they were going to clear the lines and that I was going to witness something tragic. God was on their side that day.

 

They had "The Numbers", otherwise they would not of departed.

 

Now, image the situation following an intersection departure under the very same conditions, using the same runway for departure.

 

I would imagine they wouldn't have accepted the intersection because the takeoff performance wouldn't have permitted it. Or maybe it would, with a different thrust setting: nobody except the crew on that day knows.

 

As I'm sure you know, rotation point means nothing, especially with optimised V speeds and assumed temperature derates. V1 is the significant speed as far as stopping is concerned, and again nobody except that particular crew knows what V1 was on that day or where in the takeoff roll it was called.

 

Anyway, to return to the question at hand:

 

The first thing to understand is that there are (at least) four different positions used by the FMS:

 

GPS position

Mix IRS position

Radio position

FMS position

 

FMS position is what's used for navigation, and is calculated from a weighted combination of the above positions (generally using GPS as the highest priority, followed by radio, then mix IRS). In most systems that I'm familiar with, the FMC continually takes the Radio or GPS position, compares it to the Mixed IRS position, and uses this vector to compute a "bias".

 

If Radio or GPS position is lost, the FMS adds the bias to the mixed IRS position and uses this to compute the FMS position. When Radio or GPS position becomes available again, the bias starts being updated again (hence why you may experience a map shift).

 

FMS position is usually updated with TOGA selection (which is why it is important to add any intersection data in the TO SHIFT field if required). However, it would appear the 737 does NOT update the FMS position in this manner if GPS is available.

 

Therefore, my only conclusion is that, as the FCOM states, the TO position update is inhibited with GPS available. However, the FMS will still be (continuously) using the GPS position to calculate a bias from the Mix IRS position. If at any stage GPS position is lost, this bias will then be added to the Mix IRS position, negating the requirement for the TO position update.

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Simon, it's not just the 737 that inhibits it. The 777 also inhibits TOGA position update with GPS updates enabled. With GPS updating the one off TOGA position correction simply isn't necessary.

Kevin that is correct, the error will be corrected after DME-DME updating is established.

Vernon, it is not correct. When you hit TOGA the FMC position is supposed to be corrected to the runway threshold plus any correction you entered.

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"OH, but that could never happen to ME!"  ;-D

 

I am one of many excellent pilots that will use my excellent and widely accepted poor judgment and accept an intersection departure and save time. After all, nothing ever happens and we save time! We don't risk a thing! Forget about the how many meters of runway that will be to our back on departure and of no use to us should we experience an encounter that would have made those however many meters of now useless runway useful. ;-(

 

Forget about the passengers in the back who are counting on us to make good decisions up here to make sure that they are as safe as possible. After all, we are going to save time. :-D

 

The box says that I am required to have 3,649' of runway for our conditions for this departure. My having 4,370' of runway to depart from when I can have 7,600' of runway is just fine. And, we will save time. :-D

 

Now, that's what I call using Great Pilotage Decision Making. Whether you are a student pilot or a multi-thousands of hours pro jetjock. :-D

 

Aviation is dangerous just on it's own merits. Made even more so by so many who have come to believe it okay to do what everyone else does. In spite of the tools we have available to minimize our exposure to the inherent dangers involved with aviation. :-I

 

And, made even more more so when I take to the skies! That is why it is mandatory that regulatory entities issue NOTAMs prior to any flight I make. :-/

 

 

 

 

I have never had ATC get upset with me because I didn't accept an intersection departure. Maybe some day they will. They would get mad at me for contacting the tower when I was told to monitor the tower frequency. Especially at an airport in Denver.

 

No thanks, I'll wait my turn. No penalty box is a good place to find yourself in. But I'll gladly accept the penalty box for the lost time over the penalty box when your poor cockpit judgment has to be explained as to why you went off the end of the runway.

 

But, going off the end of the runway following an intersection departure never happens.....So, "what difference at this point does it make"?

 

Maybe me just being lucky. In the hundreds of commerial airline flights that I have been subjected to, not once have the flight crews performed an intersection departure. We always departed from the proper departure point of the runway....using the full length of the runway. Thankfully, they weren't in a big hurry. I know I wasn't.

 

 

 

 

"Yeah - I'm a huge proponent for safety, but...."

 

Do you ever have a gut feeling or a shiver up the spine prior to doing something wrong or when making a poor judgment call? That's your "conscious safe decision-making" gnawing at you telling you to reconsider what you are getting ready to do. Safety is compromised when you cut corners.

 

You think "outside the box" to solve problems. Thinking "outside of the box" that creates a potentially bad situation is counterproductive.

 

".......black and white safety rules....." are not black and white. They are as red as the spilled blood that brought them about in becoming rules.

 

On a hot and humid day here, I watched a Diesel 9 depart to the east from where we based our aircraft. They started their roll from the proper departure point of the runway, black smoke trailing. The aircraft rotated with just a little less than 400' remaining of a 7,600' runway. They crossed over a set of powerlines by no more than 50'. Prior to crossing the powerlines, I didn't think they were going to clear the lines and that I was going to witness something tragic. God was on their side that day.

 

They had "The Numbers", otherwise they would not of departed.

 

Now, image the situation following an intersection departure under the very same conditions, using the same runway for departure.

 

As for the flight schools, most of the instructors have less than 400 to 500 hours, and their students are following their instructors guidance. This is not a jab at instructors as everyone has a starting point somewhere. Even most experienced instructors will tell you about one or two things they did when they were "young and dumb and ...... " that they wouldn't even consider doing after they learned otherwise.

 

At a major airport here in the U.S., during non rush hour arrivals and departures for the field, a flight instructor and his student were struck by a jet and killed. They were the only aircraft waiting to cross the active runway when the jet on departure, following an engine power loss prior to V1, left the runway and hit the small single engine aircraft.

 

There was very little arrival and departure traffic out of this airport during that time.

 

So, exposure was .....?

 

For the instructor and the student, exposure was 100%. 100% during a slow period of arrivals and departures.

 

What most people don't remember in the rush of things is that our Takeoff Performance is predicated on optimum performing powerplants and braking systems. Basically, new equipment.

 

And, for those Takeoff Perfomance numbers to be of any good, even with a "Hey, we have a problem!" momentary time lapse, the aircraft has to be relatively new or kept in relatively new condition. After all, test pilots who knew just when and what was going to occur produced those numbers, flying new equipment.

 

Sitting in the cockpit, barreling down the runway at "OH TOO FAST", having a failure, recognizing the failure, and responding to the failure with remaining runway in front of the nose of the aircraft can be quite the event. Even more the event when you very rapidly see the end of the runway coming and you still are going too fast. "OH, if only I had just another 500 to 1,000' more runway!"

 

So, having said the prior, every aircraft you fly is in nearly "new" condition. No worn brakes, bad cylinders, no hydraulic problems, etc. A lot of folks wish they could say the same.

 

Even more during these times. What with economics being the issue and most commercial operations cutting back on maintenance to save money. And, its okay now, more than ever, to perform intersection departures?

 

Next commercial flight you get on, take a look into the cockpit and view the "INOP" stickers placarded around the cockpit. They are legal by regulation and maintenance can be deferred for whatever time frame the company has had approval for. But, quite the confidence building view and the nice warm and fuzzy feeling as you go barreling down the runway following an intersection departure.

When all heck breaks loose and something happens, when meeting with your local aviation authorities and explaining YOUR outstanding command decision making procedures as justification for cutting corners, remember my "private pilot level" advice about "runway behind you".

 

Cheers,

 

QUOTE:  "Don't be a show-off. Never be too proud to turn back. There are old pilots and bold pilots, but no old, bold pilots."     E. Hamilton Lee, 1949

 

Jim Wilkerson

ATP Rank: MAJOR OVERDOER

Sir, respectfully: please allow me to point out that I've been flying airplanes for 20+ years, I've been at the airlines for 17 years, and I've been a 121 check airman for several years. I'm absolutely always willing to consider your input (or anybody's), and I'm absolutely always willing to learn new things and/or reevaluate my position. All that being said, I'm not sure I need a lecture on threat and error management / risk mitigation techniques on a flight sim forum ;-).

 

You make many false assumptions and deductive leaps in your post. For instance, 121 takeoff data IS factored, and by a significant margin. Your statement about "test pilots, brand new airplanes, no margin" etc is something that I've told many students as well because it IS true in the GA world, but not 121. Reaction time IS factored in our procedures. There IS a margin. No credit is taken for reverse thrust in an abort. Etc etc. In your DC9 example... they may have had "the numbers", but they certainly weren't GOOD numbers, or something else went wrong that should probably have prompted an abort (misconfig, improper thrust setting etc) because good numbers are such that they would have cleared the obstacles single engine by a greater margin than that.

 

If you've truly never done an intersection takeoff on an airline flight, then you've not spent much time flying out of ORD (28R at EE or WW, or 32L at T10) or EWR (22R at W) or any of the large hub airports with intersecting runway configurations. I'm not in any way exaggerating when I say that this is normal, every day, all the time. I imagine there's somewhere online where you can listen to tower comm at these airports; give a listen if you don't want to trust me. The only flights you'll hear request full length are an occasional heavy that's too loaded to get numbers for the intersection.

 

It's like Spin said; it comes down to managing risk to an acceptable level. There's no reason to refuse an intersection departure at these airports and gum things up when you are perfectly able, per very conservative data, to either abort or continue single engine at V1. If the data were not conservative, if it were not available for that intersection etc, then that would be different. If the weather were hellacious; if you had reason to doubt reported FICONs or suspect possible windshear etc, then THAT would be different. But on a normal day, why would you want to subject yourself to crossing runway ops by demanding full length? What's the bigger threat: crossing runway ops, or departing 9,000 feet of runway with good conservative numbers?

 

We routinely operate off 6,000 ft strips. Before we retired the -200s (which I would dearly have loved to fly), we did a whole bunch shorter than that. Should we infer that, since a 9,000ft remaining intersection departure at ORD is apparently unsafe, we should inform the residents of these communities that they're now off-gridders? ;-)

 

I would never fault anyone for being conservative and taking their time. But, again like Spin said, demanding full length at some of these airports exposes you to more significant threats than taking the clearance. The safety record in 121 is clear - intersection departures with good data are not really a significant threat, but crossing runway ops are. Unplanned ground delays that eat fuel and drive crew fatigue are. Etc.

 

I don't mean to sound overconfident or arrogant about this stuff because I'm most definitely not. It's just the world I know. I don't guess I'd be quick to disparage decisions made in that world if I were unfamiliar with aspects of it.

 

And, finally, on-topic: I have no idea about the original question. My assumption would be that GPS data overrides all other position correction because I've never been told otherwise!

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Vernon, it is not correct. When you hit TOGA the FMC position is supposed to be corrected to the runway threshold plus any correction you entered.

 

But isn't that what i mentioned at the start? If GPS is enabled TOGA update is inhibited

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