Selecting VS zero during turbulence in cruise
 

Have seen some guys do it for turbulence approaching moderate and heard of it being done in greater turbulence.

Any opinions?

In principle, it sounds sensible and it would reduce excessive speed variations as the aircraft wouldn't be climbing or descending to maintain altitude.

I would say you are much much more likely to then bust your level, which is probably a greater risk IMHO.

What about ALT HLD?

Why would you want to select v/s in the cruise?
Normal FMA's for the cruise are SPD / LNAV / VNAV PATH, speed is controlled by the auto throttle.
If, as you say people select v/s in the cruise the FMA's will be SPD / LNAV / V/S,
speed is still on the auto throttle.
All you are going to do is encourage altitude excursions.:=:=

Just go with the manufacturer's recommendation.

What plane are you talking about, anyway? If it's a 737, CWS and Turbulence N1 for severe. Otherwise, giddyap.

One could also argue that it would be naughty to do so in RVSM airspace, since the automatic height keeping feature required of RVSM is not being used. Trivia, I know, but I thought I'd get it in first :}

Might work for a non-FMS airplane like the 747 Classic, but I would NOT do it on an RVSM certified FMS airplane. As others pointed out, VNAV or ALT HOLD would be preferred, with VNAV MUCH preferred over ALT HOLD.

If you are in V/S mode, which FMS is now helping you out?

For SEVERE turbulence in the 744 Boeing sez:

Otherwise use normal autothrottle and autopilot procedures.

Few of us encounter severe turbulence on a regular basis, so normal procedures should be the default.

That shows how important knowing your own aircraft is. The 737-800 has a sentence in the Performance/Limitations manual specifically warning against using ALT HOLD during turbulence.

Airbus has a 'soft altitude mode' which ALLOWS vertical changes of +/- 50’ to control speed excursions rather than hosing the engines up and down via the autothrust. This is allowed in RVSM and saves fuel:

".......The soft altitude mode corrects minor deviations from the Mach target by allowing a ±50 ft variation from the CRZ FL. This feature improves fuel efficiency and passenger comfort and minimizes the changes in thrust......"


Why would you want to hold your altitude rigidly in turbulence? - surely more stress on the airframe?

It's not a matter of holding altitude "rigidly", but returning to the target altitude after an excursion.

On the type I fly (no auto throttle), cruise airspeed is close to Vne. Flying with ALT selected in turbulence will result in what can be quite an alarming phugoid flight path and a Vne excursion, or at least an airspeed audio warning, which is recorded in the data base.

VS mode is relatively benign.

It's a matter of knowing your aircraft, rather than a "one size fits all" answer.

Selecting VS zero during turbulence in cruise
 

Sorry, off topic, but what on (above) earth does VNAV / FMS have to do with RVSM? Why is VNAV *much* preferred over ALT HOLD?

post deleted and user banned.

I am still shaking my head ..... are you quite serious ?

JT

To Framer: if you cannot fly Alt HLD in turbulence, you assume an Operational FMS is mandatory? And then please explain me the difference in autopilot and A/T input, between VNAV ALT and ALT HLD mode.

You can fly ALT HOLD in turbulence. I was referring to severe turbulence as the poster before me had been referring to the 744 procedure for severe turb. Apologies if I mislead you. I'm not sure what the reference to the FMS is about, I don't assume an operational FMS is mandatory, I'm not sure why you'd think that.
Have a good day, Framer

The only option to fly in cruise maintaining altitude is ALT HOLD or CWS, VNAV ALT requires an FMS.

I never tried selecting a different altitude in the MCP, and then selecting V/S 0.

What I wonder about is what different servo inputs come from the different modes, in order to smoothen the ride.

Yip, three options. Boeing recommend CWS in the 737-800 if the turb is severe. Apart from that , any of the three is fine as far as I can see.
I imagine that once the turbulence is severe, it is not about smoothing the ride, it is about minimising the stress on the airframe. Correct me if I'm wrong.
In VNAV PATH the servo inputs are made to move the elevator to return the aircraft to the selected FL. The same for ALT HLD. In doing this there is stress on the airframe/flight controls. In CWS the servos would be holding attitude which would require less input and therefore less stress than the other two scenarios.
The above is just how I imagine it, I would be very happy to have someone with more knowledge correct me.
Cheers

Framer, if I say 'smoothing out the ride', I am talking in this case about airplane stress. This topic revolves around that.

Actually I am more inquiring about Intruder's comment, where "VNAV is much preferred than ALT HOLD". Why would this mode do anything better to reduce the stress on the aircraft?

CWS is another topic.