VNAV or FLCH during descent? What's your preference?
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VNAV or FLCH during descent? What's your preference?
VNAV, especially at low altitudes seems a bit spotty so Flch is usually a good antidote. This is my third post here, and I'm quite new here, a young buck just curious. This was a question I've always wanted to ask professional pilots especially in the terminal areas. Is FLCH usually used for descents as it gives a nice smooth ride for the passengers as opposed to VNAV on various occasions?
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The nice thing about VNAV is that it complies with altitude constraints down to the one set in the MCP (provided descent winds are as advertised), whereas FLCH just looks at the MCP.
When there is speed control however, VNAV with speed intervention complies better, managing the profile with thrust or speed brakes.
Personally for complicated STARs I stay on VNAV. If going off the planned track I.e. Hdg Sel iso LNAV, then I'd use FLCH.
When there is speed control however, VNAV with speed intervention complies better, managing the profile with thrust or speed brakes.
Personally for complicated STARs I stay on VNAV. If going off the planned track I.e. Hdg Sel iso LNAV, then I'd use FLCH.
On some types, VNAV is either not allowed to be used for procedural reasons or not used due to its rigidness / excessive accuracy, if I may call it that way. On the F70/100, the VNAV, while allowed to be used, planned a steep high speed idle descent that met all the restrictions at fixes, be it airspeed or minimum/maximum altitude/FL. All fine and dandy, but then along comes real life in form of ATC with its speed control and unexpected (to the computers, that is) instructions in the form of shortcuts, often cutting the distance to touchdown short by quite a margin while requiring massive speed reductions that easily made one drift way above the calculated profile. In such cases, it usually was the speedbrake to the rescue, which was less than well liked due to passenger comfort reasons (let me just say, it is super effective and basically just has two positions IN and OUT). So LVLCH or vertical speed was the preferred mode, manually keeping at or below the calculated VNAV profile.
On the DH8, the VNAV mode often results in nasty pitch oscillations while the autopilot a bit clumsily chases the profile at higher speeds. Not too nice for passengers and especially the flight attendant sitting at the rear end of the long tube who is a bit prone to getting seasick back there on occasion, so use of this mode is restricted to the final descent of managed FMS approaches (any kind of non precision including RNAV approaches under certain conditions and if available in the database). Vertical speed mode is most often chosen, but LVLCH mode (going by the name of IAS on the Dash) is available as well.
On the DH8, the VNAV mode often results in nasty pitch oscillations while the autopilot a bit clumsily chases the profile at higher speeds. Not too nice for passengers and especially the flight attendant sitting at the rear end of the long tube who is a bit prone to getting seasick back there on occasion, so use of this mode is restricted to the final descent of managed FMS approaches (any kind of non precision including RNAV approaches under certain conditions and if available in the database). Vertical speed mode is most often chosen, but LVLCH mode (going by the name of IAS on the Dash) is available as well.
My employer worships at the church of VNAV, and thus we are paid to use it when available. Once the path starts getting silly, or you are vectored away from it, straight to VS or Level change as appropriate.
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VNAV, in a capable airplane, especially at low altitudes, can be a life saver. Just think how much less "difficult" a visual approach to SFO 28L would be if it followed the RNAV approach profile, using LNAV and VNAV, with the autopilot engaged until minimums...
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Tu.114,
If you still fly the Dash (or for those who do and find this more than frustrating), you can put the aircraft into PITCH when the FMS gives you the two minute TOD warning. Set the PF altimeter to that given for landing, then arm the VNAV (Don't change the one on the PNF/PM side to comply with any upcoming FL restrictions). Once the FMS gives you the actual TOD warning, power back to about 70 to 75% TQ. Seems to clear up those oscillations for both the -100 and -300 airframes (assuming the wind doesn't change drastically during descent). This will allow you to ride the pole (within 10 knots).
For the -200 airframe, it's a little bit more of a game considering how close to the pole you're already flying, so you end up having to use 20 - 25% TQ to begin with.
In either case, don't change the altimeter setting while connected to VNAV. Switch over to VS or IAS, make the change, then VTO on the FMS about half a million times to centre the VNAV bar and reselect VNAV. Every time I've seen her oscillate, it's because guys have tried to change the power as the aircraft is pitching down and then tried to change the altimeter while on VNAV.
Just starting to learn the Q400, so no tips from this bird yet - not that it's even remotely close to a classic Dash except maybe the shape.
If you still fly the Dash (or for those who do and find this more than frustrating), you can put the aircraft into PITCH when the FMS gives you the two minute TOD warning. Set the PF altimeter to that given for landing, then arm the VNAV (Don't change the one on the PNF/PM side to comply with any upcoming FL restrictions). Once the FMS gives you the actual TOD warning, power back to about 70 to 75% TQ. Seems to clear up those oscillations for both the -100 and -300 airframes (assuming the wind doesn't change drastically during descent). This will allow you to ride the pole (within 10 knots).
For the -200 airframe, it's a little bit more of a game considering how close to the pole you're already flying, so you end up having to use 20 - 25% TQ to begin with.
In either case, don't change the altimeter setting while connected to VNAV. Switch over to VS or IAS, make the change, then VTO on the FMS about half a million times to centre the VNAV bar and reselect VNAV. Every time I've seen her oscillate, it's because guys have tried to change the power as the aircraft is pitching down and then tried to change the altimeter while on VNAV.
Just starting to learn the Q400, so no tips from this bird yet - not that it's even remotely close to a classic Dash except maybe the shape.
+TSRA,this might well work also on the 400. Torque values may vary of course. But it is an awful bit of extra work just to use that VNAV when one can simply use IAS or V/S instead. And besides, it seems not nearly compatible with our SOPs here.
The more elegant method on the 400 seems to be quickly calculating a 3 degree descent and flying it non-managed. This seems to result in a smooth, lowish workload descent path.
The more elegant method on the 400 seems to be quickly calculating a 3 degree descent and flying it non-managed. This seems to result in a smooth, lowish workload descent path.
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To be honest it really depends, VNAV takes care about all those limitations, be it speed or altitude or the aircraft envelope. However sometimes it is just better to use V/S or Level Change. All are tools and as a professional i use pretty much all the tools available to me. SOP tells me to use VNAV and for the most part it works out quite nicely until we get into the TMA where ATC starts to interfere with vectors and the like.
We do fill the descend forecast page to get a better VNAV path, biggest part is winds which are downloaded together with cruise winds whenever we do download those, guessing the altitudes for engine anti ice is another big part and that is quite a bit more fuzzy. Of course taking into account expected short cuts isn't bad either, and of course it helps that we use CI 2 to 8 currently which leaves a lot of wiggle room if one is high on path. Doesn't hurt to have a mental picture of where you are, how high and how many miles you need for descent either, that is basic airmanship and trained extensively in LIFUS.
We do fill the descend forecast page to get a better VNAV path, biggest part is winds which are downloaded together with cruise winds whenever we do download those, guessing the altitudes for engine anti ice is another big part and that is quite a bit more fuzzy. Of course taking into account expected short cuts isn't bad either, and of course it helps that we use CI 2 to 8 currently which leaves a lot of wiggle room if one is high on path. Doesn't hurt to have a mental picture of where you are, how high and how many miles you need for descent either, that is basic airmanship and trained extensively in LIFUS.
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My experience is whenever I input perfectly accurate winds into descent forecast page the plane tend to slow down below Econ descent speed to the point where thrust is added. Entering altitudes for TAI also seem to have relatively little relevance, in my humble opinion. Personally I normally leave the page empty apart from the destination QNH, which if very high or low can affect the profile significantly.
Where I go we are mostly on vectors from very early on which make V/S or Level Change more appropriate. Whenever not following the magenta line (LNAV) I go out of VNAV, even though it can be used if the vector somewhat parallels the magenta. VNAV can be a fantastic tool for optimizing continuous descents but i have also seen it done pretty weird calculations so running a mental gross error check is a good idea.
Where I go we are mostly on vectors from very early on which make V/S or Level Change more appropriate. Whenever not following the magenta line (LNAV) I go out of VNAV, even though it can be used if the vector somewhat parallels the magenta. VNAV can be a fantastic tool for optimizing continuous descents but i have also seen it done pretty weird calculations so running a mental gross error check is a good idea.
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I use VNAV most of the time. To the above poster, contrary to you I find the TAI makes a huge difference. I tested it once, I started descent without it, entered the TAI information and there was a 1500ft difference. Although it's not the easiest thing to predict and normally I can't be bothered to work it out and just rely on the x3 tables when I get closer. I typically switch to V/S when on vectors to perfect a CDA. Level change I use when I'm high and need to get down whilst on vectors. If I'm high but still far out (e.g after a shortcut) VNAV with speed intervent does a good job. Really all depends on the situation you are in.
As mentioned above, it's a good idea to use the 3 times tables for your altitude, work out the miles you need vs the miles you have, take into account the wind and need to decelerate eventually. Basic airmanship.
As mentioned above, it's a good idea to use the 3 times tables for your altitude, work out the miles you need vs the miles you have, take into account the wind and need to decelerate eventually. Basic airmanship.
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I used all 3 -- FLCH, VNAV, and V/S -- on the arrival to HKG in my 748 this afternoon. With vectors, speed control, descents in holding, and a final slam-dunk to short final, each of them was appropriate at different times. I guess that's why all 3 are installed...
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Okay good. VNAV is used rudimentary and then as you are flying into the terminal environment, it is necessary to use all 3 methods pertaining to the conditions that are presented. Makes sense! I never really knew what methods the professionals used on most accords. Thanks for clearing that up.
Now I only happen to be a student pilot in training, but currently use X Plane (laminar research) to exercise my instrument navigation and procedures. Having a flight simulator is quite useful when you're not in the airplane but have that urge to fly!
Now I only happen to be a student pilot in training, but currently use X Plane (laminar research) to exercise my instrument navigation and procedures. Having a flight simulator is quite useful when you're not in the airplane but have that urge to fly!
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Why not use all four?
LVLCH, V/S, VNAV & FPA are descent tools (with or without A/T and airbrakes). Each has a function. They also each have their own limitations and traps. And you use them accordingly. But to limit the use of a function because a whim or dogma is Luddite mentality.
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OP asked about preference between VNAV & FLCH. But as V/S has been mentioned, it is useful when ATC requires a minimum descent rate, or when cleared for the approach after vectors we find ourselves well below the ideal profile.
As has been mentioned, they are all tools to be used judiciously.
As has been mentioned, they are all tools to be used judiciously.
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It really depends on what phase of flight you're in... VNAV is nice when you have speed restrictions and big stars to stroke .. When you're being vectored away from your path FLCH is always a good choice speacially when using Flaps + FLCH..to get those CDA's nicely..
