Changing from VNAV -> SPEED

[Jeff Nielsen 1,385]

I've tried it a couple of times using VNAV to 'try' and descend below the published minimums for a runway with a GPS approach and no ILS approach using real world WX.The alititude restrictions on a GPS approach seem to end quite aways from the runway and it seems about 1500 AGL or so. This is where the chart usually shows LNAV only from there on. Likewise the missed point is about 1 mile before the start of the runway.I haven't been able to do it yet after a few tries, but this post on using the speed intervention while still in the VNAV mode has increased my knowledge alot and just might help me cheat a bit. hehe. ABQ has a GPS appr to a runway (can't remember which one) I used becaused the winds were just to high to try a normally used runway served by an ILS.I'm sure it's completely illegal in the real world...LOL, looking at the real world GPS Appr. charts anyways.For simmers this could help when flying into smaller airports with the 737 where no ILS exists. In other words when making up flights. :-)

[Qavion 19]

"The IRUs are constantly being updated by GPS (twice a millisecond, or some such interval), so the airplane is always "really" using a GPS present position (PP)."This is incorrect, Sam. The FMC position is being updated by GPS's. The IRS's only use GPS position when you manually insert it at the start of the flight via the FMC CDU (or whatever lat/long reference you think is appropriate at the start of the flight.... Gate lat/long ,etc). The IRS's then, in flight, provide pure IRS position information to the FMC's. The GPS data (along with the other nav radio radio data) is fed directly to the FMC's on different data busses.Cheers.Q>

[macpac22 0]

Maybe you guys might be able to clear something up for me in regards to VNAV operations.CLIMBOk lets take a typical flight with a SID. Lined up on the runway i will have LNAV and VNAV armed.After take off and autopilot switched on, the aircraft will follow the SID. At what point would thecaptain switch from VNAV to Vertical Speed (V/S) to make sure the aircraft climbs to meet the height restrictions of the SID. Would it be right after take off or would the captain monitor the initial climb and use the Green Arc in the Navigation Display as a guide?Also if i continue to use VNAV instead of V/S, when i reach 10000ft the aircraft will nose down to accelerate to climb speed and then pitch up to continue the climb. If that was to happen in real life all the passengers would get sick :) my question is that during a climb do you guys use VNAV or do you prefer V/S for a gradual climb with no sudden pitch up or nose down movements?DESCENTok so ive reached my T/D and start to descend in VNAV. What i normally do is descend down in VNAV until i reach Transition altitude of 18000ft. I would then switch from VNAV to V/S and press the SPD button and control my speed and monitor my descent using the green arc in the ND. Again id like to know the procedure that you guys would fly in this case.Im sorry i cant be really specific im in work at the moment :((

[Qavion 19]

"At what point would thecaptain switch from VNAV to Vertical Speed (V/S) to make sure the aircraft climbs to meet the height restrictions of the SID."If you have to use V/S, then there is something seriously wrong here, Paul. As mentioned in previous threads, VNAV gives you full (or derated) climb thrust and you have to accept whatever climb rate results. If you're not meeting height restrictions, then the aircraft is too heavy and/or your climb derate is set too high. Using V/S is not the answer. AFAIK, you are not going to get a thrust increase by switching to V/S... and you risk a stall if you set the V/S too high (no speed protection in this mode).Another factor in you not meeting your altitude restriction could be your choice of flaps for takeoff. Flaps 10 means a longer takeoff roll and more chance of bumping into something at the end of the runway, but it will give you faster acceleration and greater terrain clearance in the long term. Flaps 20 means a shorter takeoff roll, less chance of bumping into something at the end of the runway (or near to the end of the runway), but slower acceleration and poorer long term height gain (and you may bump into that mountain several miles from the airport).Note that the FMC will tell you if you're not going to meet the height restriction (in the form of a scratchpad message).It's very unlikely that your passengers are going to get sick if you climb too quickly. The automatic pressurization system will try to give you a comfortable cabin px rate change... and unless the pax are wandering around the aircraft just after takeoff, it's unlikely that airplane body attitude will make them slide up and down the aisles :(At this point, I can't say what's causing your problem (too heavy, high derates, wrong flap selection, etc).Cheers.Q>

[Guest D17S]

Like Q said. Let Vnav do it all. It will take you up and get you down. V/S is a hazard. A real accident just waiting to happen. For descent, once you are within 25 miles of your runway, tap the MCP airspeed knob. You can then dial in your airspeeds you bring in the flaps and gear to your final Vref airspeed. Vnav is shooting for your RW's field according to barometric altitude. This the only altitude reference Vnav has to go by so make sure you have reset your altimeter to field elevation . . . otherwise Vnav will "miss." Disco the AP at 10 feet RADIO altitude feet, flare and rollout.Of course you realize this is all just for fun and and maybe to glimps the future of aviation. This is not a proscribed, ReaL Life procedure . . . but it's amazing to see a near RL airplane (the PMDG model) do it.

[macpac22 0]

thanks for clearing that up guys! :)

[michal 44]

> V/S is a hazard. A real accident just waiting to>happen. But we know that real pilots use it sometimes. Captain Alan Carter from VA (the one from this famous ITVV DVD) uses it quite often on climb out and descent as anybody who watched this video would testify. And he would probably assure you that he knows what he is doing and no harm is coming ;)Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[macpac22 0]

Thats why i asked because i watched that DVD the other day and this is what happens:Take off from runway 27R with a WOB2F departureAfter rotation:At 800ft "Left Autopilot to Command"THR REF | LNAV | VNAV SPD301 Degrees inbound to Burnham (which is 3000ft or above)"SPEED - VERTICAL SPEED" is called by AlanSPD | LNAV | V/Sreduces rate of climbManually dials in speed of 300knots with a V/S of +1100Passes Burnham no problem with 3000ftContinues towards London no change in V/SAlan says "London 10miles 4000ft no problem"so obviously alan used V/S to manually monitor the climb for the SID.They are then given a radar heading after clearing the SID at 6000ftThey are at 6000ft and atc gives them clearance to FL100Alan says "into VNAV - THRUST REF VNAV SPD and ill keep the speed at 300knots"VNAV is from that point on to climb to FL330

[michal 44]

Exactly.I figure if Boeing did not want pilots to use V/S it would simply remove this flight mode altogether.Pilots are paid good $$ to know how not to stall aircraft so having V/S is just another tool in their tool box. It reminds me a bit about the "goto" in the C programming languge. Technically it is a bad practice to use goto in your program however there are some isolated cases when it is fully justified. Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Qavion 19]

"And he would probably assure you that he knows what he is doing and no harm is coming ;)"In this case, the pilots are using V/S to limit their climb rate. The original poster didn't make that quite clear. There is no harm in using V/S for slow climbs like this (and descents). V/S is used from time to time(I'm not arguing that) e.g. slow step climbs in cruise so as not to disturb sleeping passengers. V/S is also a basic tool which can be easier/quicker to use sometimes rather than reprogramming the FMC to meet height restrictions. But I'm sure there are FMC-savvy pilots out there who could have used the FMC to do the same job as in the VA video (workload permitting).There are 744 pilots out there who would laugh at you for using V/S... and pilots out there who would laugh at you for using the FMC for basic manoeuvres... It all depends on the company/pilot background/etc. ;)Cheers.Q>

[WebMaximus 372]

Always very interesting to read your posts Q, thanks for sharing all the info and knowledge!

[michal 44]

>Always very interesting to read your posts Q, thanks for>sharing all the info and knowledge!Ian doesn't post very often but when he does he is 100% right.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest D17S]

And to add a little more color . . . The 767 production run began installing GPS in the 767s around 2000. My company has 767s both with and without GPS. It was interesting to read how Boeing describes these interfaces. First, Boeing states,