B744 A/P in V/S during turbulence

[hamil]

Hi,

In cruise I've noticed that some pilots prefer to select zero on vertical speed mode in case of turbulence instead of keeping the A/P in VNav Path or Alt Hold.

They say the airplane rides smoother in such ups and downs.

Does it make sense?

[eckhard]

VS may well provide a smoother ride but it won't maintain your assigned FL.

Consider this: You are at FL350 and you select VS 0 due to turbulence. You hit a gust and the aircraft drops 100ft. The VS mode reduces the descent rate to zero but you are now maintaining FL349. The process repeats. And again. Before long you have descended 400ft. What about RVSM?

If the turbulence/MTW is really so bad that holding your FL is abandoned in order to stop the aircraft from stalling/overspeeding, the best solution might be to disconnect the AP, set a sensible N1 or EPR (95% or 1.5) and hold the pitch attitude at about 3 degrees nose up. The IAS or mach no should then stay reasonably constant while the FL varies up and down. Might want to tell ATC what you're doing and keep an eye out for other traffic.

[bigduke6]

I have seen it done that way, but it does not make sense to me. But I have used V/S for climb during turbulence, as recommended. I have never seen a Boeing reference recommending using V/S in cruise for turbulence. But perhaps someone will present one?

per FHB:
"Severe Turbulence
The recommended procedures for flight in severe turbulence are summarized
below.
Climb and Cruise
After takeoff and when established in a clean climb configuration, the autoflight
system is recommended for flight through turbulence. To reduce pitch changes as
the AFDS attempts to fly speed with elevators, climb and descend using vertical
speed (speed on thrust) and cruise using altitude hold.
During cruise, VNAV and altitude hold modes each fly speed on autothrottles and
can be used in turbulence
In extreme turbulence, it may be necessary to disconnect the autothrottles.

[JammedStab]

per FHB:
"Severe Turbulence
The recommended procedures for flight in severe turbulence are summarized
below.
Climb and Cruise
After takeoff and when established in a clean climb configuration, the autoflight system is recommended for flight through turbulence. To reduce pitch changes as the AFDS attempts to fly speed with elevators, climb and descend using vertical speed (speed on thrust) and cruise using altitude hold.


Why would V/S mode be better for climb and descent?

[rudderrudderrat]

Hi Jammedstab,
Why would V/S mode be better for climb and descent?
The idea is to reduce pitch changes.
Since the AP chases the speed using elevators in the climb & descent, by using VS, the pitch is more stable. You have to accept bigger speed changes until the AT system catches up.

[8che]

Jammedstab

Think about it. Whats controlling airspeed in the Climb ? Pitch. So turbulence/speed variation can only result in further pitch changes.

Go to V/S and speed control is now instantly (and more accurately)controlled by throttles. Additionally pitch changes are reduced to almost zero resulting in a far smoother and stable ride.

exactly the same for descent if you started in FLCH or VNAVSPD. In descent if you lose airspeed with these modes while in turbulence how are you going to recover ? Increase thrust...nope. V/S mode is the answer.

V/S in cruise is ridiculous and another one of those old wives tales that somehow become unofficial fact. There are other myths too. Is anyone still peddling the old SPD window open is more accurate than FMC speed ? Hope not.

[JammedStab]

Jammedstab

Think about it. Whats controlling airspeed in the Climb ? Pitch. So turbulence/speed variation can only result in further pitch changes.

Go to V/S and speed control is now instantly (and more accurately)controlled by throttles. Additionally pitch changes are reduced to almost zero resulting in a far smoother and stable ride.

exactly the same for descent if you started in FLCH or VNAVSPD. In descent if you lose airspeed with these modes while in turbulence how are you going to recover ? Increase thrust...nope. V/S mode is the answer.



Thanks. Forgive a silly question but what about if in VNAV PATH on descent?

[8che]

Well VNAV path descent is a good question. Boeings guidance is that V/S mode is the preferred mode for turbulence but of course VNAV path is a more controlled descent in these circumstances than FLCH/VNAVSPD as pitch is controlling a desired path only. If it gets to its limit though it will automatically revert to VNAVSPD which may well occur in significant vertical turbulence/shear. Thus V/S covers all bases.