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Forecasting Arrival Barometer & Descent Forecasting

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Hello, I had a question regarding the descent phase of the flight.

 

I've just started using weather (Active Sky) and I'm coming in a bit 'hot' on my descents. To hopefully avoid other pitfalls (from threads that I've read):

 

Here's how I'm setup...

 

I'm flying from KSMF -> KLAX with a cost index of 40 and a descent speed limit of 284. I'm flying the SADDE6 STAR and my first magenta marker is 280/12000 at SYMON.

 

My typical cruise altitude is FL350, I have wind data programmed into cruise waypoints before T/D. I have wind values programmed at FL300, FL240, FL180 but I'm still a little high (according to the DES page on the FMC between 18,000 and my first restriction at 12,000. It's just about always a tailwind but in this particular instance it was 290/26.

 

I had to ride my speed brakes the whole way from SYMON to SADDE to get from 280K to 250K with only 8 nm between the two and typically passing around ~ 11,000 feet.

 

I've got, what I believe to be the correct calculation for ISA DEV and I have the QNH average for REYES & FIL (where I'm typically crossing 18,000.

 

So I have two questions that I could not find based on my experience with the two tutorial flights:

 

1.) How typical is speed brake usage on descent even with everything forecasted/entered correctly? Are there some descent/routes where it's just about a given that they'll be up most of the way?

 

2.) The transition from Flight Level(s) to Altitude cause all of sort of confusion for my FMC on this descent. I know the pressure at (approximately) my transition level and what I've been doing is putting that in prior to descent so that it appears below STD on the PFD. I noticed in the tutorial #2 flight he did not do this, so is that most likely my mistake? Follow up question to that, when do I switch the baro pressure on the descent to the arrival airport? (It doesn't specifically mention what to do on Page 80 of T1 and Page 142 of T2).

 

I think that's all the questions I have for right now. Apologize if I've written too much, I had a week plus of watching the forums and tried to address the frequently asked questions on the front end.

 

Thanks,

Jason Fearing

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1.) How typical is speed brake usage on descent even with everything forecasted/entered correctly? Are there some descent/routes where it's just about a given that they'll be up most of the way?

 

Depends on the arrival, but there are ways to get around using it.

 

 

 


2.) The transition from Flight Level(s) to Altitude cause all of sort of confusion for my FMC on this descent. I know the pressure at (approximately) my transition level and what I've been doing is putting that in prior to descent so that it appears below STD on the PFD. I noticed in the tutorial #2 flight he did not do this, so is that most likely my mistake? Follow up question to that, when do I switch the baro pressure on the descent to the arrival airport? (It doesn't specifically mention what to do on Page 80 of T1 and Page 142 of T2).

 

Overthinking it. Change it as soon as you stop hearing "descend and maintain Flight Level X," and first hear "descend and maintain X."

 

This might be helpful:

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Kyle - Heh, I've watched all your other videos but that one apparently. Alright I'll check it out.

 

Can you give me an example or two of getting around it? The most obvious thing I can think of is to use a hold at the waypoint before.

 

I've got about 50 hours in-game on the plane and I'm trying to feel very comfortable with VNAV/LNAV programming and forecasting before I go slightly more manual (V/S) but if that's the best way to fly certain descents, then I'll start clicking different buttons on the MCP.

 

Thanks!

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Can you give me an example or two of getting around it?

 

Better planning about 100nm prior to the T/D (get the wind, get the ISA DEV / pressure and enter it at least 100nm prior to give you space in case the T/D moves 'back').

Ensuring your hardware throttle is all the way back at idle.

Using SPD INTV to set a higher speed to recapture the profile if you get above it.

 

 

 


I've got about 50 hours in-game on the plane and I'm trying to feel very comfortable with VNAV/LNAV programming and forecasting before I go slightly more manual (V/S) but if that's the best way to fly certain descents, then I'll start clicking different buttons on the MCP.

 

You'll get used to it. Some people like V/S. I don't really use it that often. If anything, in a VNAV descent, I'll use SPD INTV and set the speed higher, which commands a steeper descent.

 

A lot of simmers get themselves into hot water by improperly "connecting the dots." In other words, connecting the STAR to the approach when this isn't proper in many areas. Many STARs use vectors in between the end (or near the end) and the approach.

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One thing that I like to do is start my descent about 10-20 nm from TOD. Then watch the DES page in the FMC for the V/S required. It's typically about 2,000 give or take. They say to keep the VPA below 3.0. Most of the time it gives time to level out for slowing too. If you're relying on late descents from ATC then it's a problem either way.

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The best way of making things work out is to plan the whole route with predictions. Obviously you can't always make a destination plan if you're on a long haul, but you can sometimes look at the weather trends and judge from there. Short routes can often give pretty good clues into the winds so you'll have a better idea of the runway in use as well. Otherwise it can help some to assume the closest runway (approach) is in use until you know for sure which runway is actually running. No harm in leveling out at 10,000 and staying there for a few miles.

 

As I said earlier, ATC can often make things very difficult, especially with programmed ATC or default ATC. I've never flown online but having watched a few videos of live ATC with Vatsim and even Pilotedge has shown that slowing for a clean descent can be tricky. So it's always nice to plan way ahead. The advantage to online is that you can ask and get some idea. The last few flights I've done never required speedbrakes deployment using the method I mentioned above (FMC DES page). You get a realtime V/S requirement based on speed and other factors.

 

I would say that typically pilots don't just rely on FMC path profiles in the real world. Between ATC changes enroute and various other dynamics that MCP handling is necessary.

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1.) How typical is speed brake usage on descent even with everything forecasted/entered correctly? Are there some descent/routes where it's just about a given that they'll be up most of the way?
An interesting comparison. The 737-800 requires the use of speed brakes at certain points during the descent. The 777F requires manual thrust in the 50-70% N1 range to maintain 290 to 300 knots during the descent and the A/C slows to 240 at BAYST by reducing the descent rate to 750-800 FPM.

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Thank you all for the tips/advice.

 

I currently fly offline with no ATC (my play schedule never seems to match up with the Oakland Center Vatsim team. The feedback on payware ATC is too polarizing for me to spend the money on the wrong one, right now. I went the route of scenery first, ATC later (versus the other way around).

 

I tried the V/S method last night from LAS to SMF with a 15-20 kt headwind and definitely got significantly over profile on the descent, opting for the early descent (about 15nm before programmed T/D). But I never exceeded 1600 FPM so that was most likely the problem.

 

I really want to have some degree of 'mastery' before always being logged in with Vatsim. But perhaps the payware ATC needs to be before FS2crew and GSX, so I can get a feel for some of these "wrenches" that get thrown.

 

It sounds like, from what ya'll are saying and what I'm reading/hearing:

 

- Garbage in, garbage out on the FMC. If I'm having to use excessive speed brakes I'm not supplying good data.

- Don't blindly lock in VNAV, even with data and hit go

- Be agile with descent profile management. Be prepared to jump in and out of V/S and LVL CHNG when applicable.

 

Ultimately (assuming no ATC), it's never going to be a static do X, then Y and finish with Z. But rather, what is X right now... well if it's that, then you need to do this to get to Z.

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I've not really had any problems following the FMC descent when using the descent forecast. The only time I'll run into problems is if the final point in my route is a waypoint thats on the other side of the airport, with a discontinuity to the first point in the approach. The FMC doesn't know you're probably going to take a shortcut, it thinks you're going all the way to that waypoint before turning back to catch the approach. So you should always click through your route to see how you're approaching the airport. If you ignore it, you might be 5,000 feet high when you take the gigantic shortcut of turning off the FMC route to intercept the approach.

In that case I'd probably create my own waypoint wherever I think I'm going to turn off the main route, then estimate where I need to be as far as altitude and speed.. then the FMC will get me there, and when I do get there and click off VNAV / LNAV I'll be right where I want to be.

 

It comes down to planning. 

There are a few approaches though that you just need to use the speedbrakes. The route into Minneapolis from the east puts you over a military base in Wisconsin I think, so theres some really high really fast restriction there, once you clear it theres a huge dive into Minneapolis. Theres nothing you can do but lift up the spoilers at that point.

 

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Thank you all for the tips/advice.

 

I currently fly offline with no ATC (my play schedule never seems to match up with the Oakland Center Vatsim team. The feedback on payware ATC is too polarizing for me to spend the money on the wrong one, right now. I went the route of scenery first, ATC later (versus the other way around).

 

I tried the V/S method last night from LAS to SMF with a 15-20 kt headwind and definitely got significantly over profile on the descent, opting for the early descent (about 15nm before programmed T/D). But I never exceeded 1600 FPM so that was most likely the problem.

 

I really want to have some degree of 'mastery' before always being logged in with Vatsim. But perhaps the payware ATC needs to be before FS2crew and GSX, so I can get a feel for some of these "wrenches" that get thrown.

 

It sounds like, from what ya'll are saying and what I'm reading/hearing:

 

- Garbage in, garbage out on the FMC. If I'm having to use excessive speed brakes I'm not supplying good data.

- Don't blindly lock in VNAV, even with data and hit go

- Be agile with descent profile management. Be prepared to jump in and out of V/S and LVL CHNG when applicable.

 

Ultimately (assuming no ATC), it's never going to be a static do X, then Y and finish with Z. But rather, what is X right now... well if it's that, then you need to do this to get to Z.

Every flight will have a degree of different dynamics.  Weather, weight, ATC instructions, all affect the outcome.  In reality, if ATC gives you an instruction, especially online, then you'll have time to get down.  You might be forced to deploy some speedbrake, but you shouldn't have to ride them very far.  Some approaches are more difficult too.  The default ATC always puts you too high or too far out and too low, depending on the location and your flight.  I use it VFR with a flight following when weather permits.  Otherwise file a plan IFR and don't use it unless you need it, then use it when really close to the airport.  It's going to be screwy though, it's kinda lame.

 

If you look around online you'll see that piloting a jet with VNAV takes a great deal of time to learn.  It's not just lock in VNAV and forget it.  Again, RW has ATC that knows what's going on and won't leave you lost too much.  Either way, RW pilots often rely on level change and V/S for descent.  From there it's some crafty planning with winds and such.  Some will even put in higher winds to avoid a major issue.  The newer jets are so aerodynamic and clean that they are hard to slow too.

 

Don't worry so much about your descent planning online.  Be more familiar with the overall aircraft and operations and leave the descent instructions to the controllers.  Most of them will step decent anyway, so you can slow in time.  Only worry about it if you can't meet a speed or waypoint at altitude on profile.  Then tell them. 

 

For offline, it's better to be level here and there than trying to cut two or four thousand feet off at the last minute.  And you really don't want to enter a hold for that.

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Descent planning and execution in a jet is, fundamentally, an energy management problem. You start at the top with a heck of a lot of kinetic (movement) energy, and a shed load of gravitational potential energy, and your task is to end up stopped on the runway with kinetic energy and zero potential energy.

 

Therefore, it's a little bit like a rollercoaster -- you have to manage the aircraft's total energy level and control the rate at which you convert energy from one form to another.

 

When you think about it in these terms, the interrelationship between the three variables -- speed, height and distance -- becomes clearer. More speed = more energy: 10,000ft at 35NM from touchdown is a good "gate" to aim for at 250 knots, but if you are at 10,000ft and 320 knots with 35NM to run you are no longer on profile -- you are high, because you will need extra distance to lose that extra speed.

 

The thrust levers 'add' energy -- they convert the chemical energy stored in the fuel to kinetic energy via the engines. Therefore when you take the thrust levers out of idle during the descent, you are burning extra fuel and increasing the distance required to land. Conversely, energy is 'dissipated' through drag, so devices such as speedbrakes, flaps and the landing gear cause energy to be transferred away from the aeroplane at a higher rate and therefore steepen the descent and reduce the distance required to land.

 

What this means is that coming down on the speedbrakes all the time (ignoring any ATC restrictions for the time being) is an inefficient way to fly, because it implies you could have closed the thrust levers earlier and thus saved the fuel which you used to keep the aircraft at altitude and are now trying to get rid of by deploying drag!

 

The other interesting question if two aeroplanes descend from FL350, but one maintains 230 KIAS in the descent and the other maintains 340 KIAS, which one reaches FL100 in the shortest distance? It is tempting to say the aircraft at 230kts -- because it is covering the ground more slowly.

 

However, in actuality the faster aircraft reaches the lower altitude in a shorter distance. This is for two reasons -- firstly, because drag increases with the square of speed. This offsets the higher ground speed. Secondly, the best glide speed -- the speed at which the best lift to drag ratio is attained -- is typically around the minimum clean speed, which in a B737 I imagine will be between about 200 and 220 knots. The closer to the best glide speed, the greater range will be achieved.

 

This also explains why, perhaps counterintuitively, the aeroplane needs a longer distance to descend when it is heavy compared to when it is light, assuming both descend at approximately the same airspeed -- the best glide speed for the heavier aircraft is higher and thus closer to the actual descent speed.

 

As a pilot, therefore, there are a number of tools that can be used to adjust the rate and, more importantly, gradient of descent. The first option is generally to increase the speed (with idle thrust, obviously). If this is not enough, then extra drag will be required in the form of speedbrake. If you are really high, you could consider lowering the gear (but once the gear's down, it's down -- we don't go faffing around pulling it back up again).

 

What can help is if you are getting high -- for instance, ATC delay your descent -- what you should first do is avoid putting any more energy in to the system than you absolutely have to. Therefore, it would be a good idea (provided you are not under a speed restriction) to reduce speed back towards minimum clean in level flight. This means that when you do get the descent, you then have more margin to wind the speed up and get the steepest possible descent.

 

VNAV is a useful tool, but like any computer it's only as good as the information you put in. If the lateral element of the flight plan is not representative of the route you will actually follow, the VNAV calculations will be wrong, for instance. It's important to always sanity check the VNAV profile and to be able to manage the descent yourself.

 

For this -- the rule of thumb is to multiply your altitude by 3 (e.g. 35,000ft -- drop the zeros -- 35 x 3 = 105NM), add a bit (as a fudge factor to allow slowing down -- not sure exactly of a good number for the B737, but around 10NM might be reasonable), then if you really want to be accurate you can correct for the wind (add or subtract a third of the headwind or tailwind component -- a third because a descent takes about 20 minutes, which is a third of an hour. Add for a tailwind, subtract for a headwind -- so, for example, with an average 35 knot tailwind you would add about 12NM on top).

 

 

 


Don't worry so much about your descent planning online.  Be more familiar with the overall aircraft and operations and leave the descent instructions to the controllers.

 

I would disagree with this -- controllers are not there to fly the aeroplane for you, there are there to keep you separated from other traffic. Descent planning and execution is entirely down to the pilot -- of course, ATC will probably descend you at a sensible place, but there's no guarantee and they do not necessarily know your needs! Rather than waiting for ATC to give you a descent instruction, you should know when you need to descend and if you don't get the instruction by then then you need to be asking for it.

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Meant to add -- set yourself some 'gates' before TOD so that you can gauge your progress. As a rough guide, I tend to use FL200/ECON speed/75NM, FL100/250kt/35NM, 3000ft/10NM/flap 1 in the Airbus -- will be roughly similar for the NG I imagine but someone more experienced with the 737 could confirm.

 

As a quick 'how-goes-it', distance x3 gives you the height you should be at (so 25NM x 3 = 7500ft above the airfield, for instance).

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The best way of making things work out is to plan the whole route with predictions. 

 

 

As a quick 'how-goes-it', distance x3 gives you the height you should be at (so 25NM x 3 = 7500ft above the airfield, for instance).

 

Thank you both for your detailed and thorough responses. It's all starting to make a lot of sense. I appreciate it and you've given me some great 'take home' advice to use on my next flight.

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So I've now done 5 trips on the original route in question and definitely using SPD INTV has helped considerably when necessary. The last 3 trips were 100% speed brake free.

 

One last question I had and we can button this up.

 

I'm 280/12000 and I need to slow to 250/10000 at an upcoming step on my trip. Do I descend to 10k and then slow or slow and then descend to 10,000? Or is weather/wind specific (the answer would depend on conditions).

 

Kyle... in watching your supplemental tutorial v2 (around the 1 hr, 28 min) mark you're descending to 6,000 and then give yourself instruction to slow to 220. But you don't slow until you reach 6k versus enroute to 6k. Is that typically the way it's done?

 

Get to the target height and then slow? I know that's how the FMC calculates it, typically (the decel point right before the target waypoint).

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Or is weather/wind specific (the answer would depend on conditions).

 

Depends on the situation. The 10/250 thing is more of a slow before reaching kind of thing, so you descend, reduce the descent rate just prior to 10 and reduce speed to 250. In cases where I need a steeper descent, I might slow down before going down where I can use the speed reserve for an increased descent rate. If I'm low enough, slowing down before trying to descend may also help me get flaps out to aid said descent.

 

 

 

Kyle... in watching your supplemental tutorial v2 (around the 1 hr, 28 min) mark you're descending to 6,000 and then give yourself instruction to slow to 220. But you don't slow until you reach 6k versus enroute to 6k. Is that typically the way it's done?

 

The instruction was "reaching 6000, reduce speed 220 knots or less." In other words, "upon getting to 6000, reduce speed to 220 or less." I was distracted by not properly setting up the VOR info once I got to 6000 though, so my action was a little delayed.

 

EDIT: Re-watched that one segment (thanks for the time stamp on that - helped me from having to search for the reference) and noticed how weak my old computer was...man...ouch... :lol:

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