Selecting VS zero during turbulence in cruise

[JammedStab]

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

Any opinions?

[Field In Sight]

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.

[haughtney1]

What about ALT HLD?

[sleeve of wizard]

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.:=:=

[ImbracableCrunk]

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.

[Pontius]

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 :}

[Intruder]

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.

[latetonite]

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

[Intruder]

For SEVERE turbulence in the 744 Boeing sez:

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 (SPD on thrust) and cruise using altitude hold.

Otherwise use normal autothrottle and autopilot procedures.

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

[framer]

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.

[Uplinker]

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?

[Intruder]

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

[ShyTorque]

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.

[JeroenC]

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

[Fair.Pilot]

post deleted and user banned.

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

JT

[latetonite]

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.

[framer]

To Framer: if you cannot fly Alt HLD in turbulence, you assume an Operational FMS is mandatory?
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

[latetonite]

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.

[framer]

The only option to fly in cruise maintaining altitude is ALT HOLD or CWS, VNAV ALT requires an FMS.
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.
What I wonder about is what different servo inputs come from the different modes, in order to smoothen the ride.
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

[latetonite]

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.

[Inbound On Descent]

It was always drummed in to me that during turbulence one should attempt to hold an attitude, not an altitude - perhaps the basis of the original question.

I've flown two jet types with an FPA mode, where selecting will hold the current angle - not sure about Airbus or more modern Boeings. if that doesn't work, start with FPA=0 and go from there...

Yes / No ?

[con-pilot]

It was always drummed in to me that during turbulence one should attempt to hold an attitude, not an altitude

Correct. Except that should read extreme turbulence.

Also, it is my experience that a very few number of pilots have ever experienced any extended period/s of real/actual extreme*turbulence.

Just because an auto-pilot disconnects, does not mean that one is in extreme turbulence.


* Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.

[FullWings]

Also, it is my experience that a very few number of pilots have ever experienced any extended period/s of real/actual extreme turbulence.
I’d second that. The methods described are for when you can’t read the instruments for the shaking and the stall and overspeed warnings are going off at the same time.

Spilling tea mid-Atlantic and wondering if the crew should be sat down or not doesn’t come close. ;)

I have only encountered what I would describe as severe turbulence in a jet transport once in 20+yrs of flying and I do not wish to encounter it again: it was a fight just to keep the brown and blue the right way round, nothing else was controllable.

The effect of V/S zero is probably type dependent - on the ones I’ve flown, if you were displaced from your cruising level they would probably not attempt to return to it, just try to fly level at the new datum...

[con-pilot]

I have only encountered what I would describe as severe turbulence in a jet transport once in 20+yrs of flying and I do not wish to encounter it again: it was a fight just to keep the brown and blue the right way round, nothing else was controllable.


That would be it, only managed to encounter such twice in my 42 year, 21,000 hour career. One when as co-pilot in a Jet Commander* when complete control was lost at FL 390, when our radar failed traversing a line of thunderstorms, recovering around 12,000 feet when we were thrown out of the side of the storm inverted.

Second time was due to CAT in a 727. I lost 4,000 feet before I could regain complete control of the aircraft. I had some cabin crew injuries then, as there was no warnings at all. It was smooth as could be in total VMC and we ran into a brick wall**. Well it seemed like it and as you said, it was all I could do to keep the greasy side down, barely.

* Some may criticize Jet Commanders, but one thing about them, they were built like brick :mad: houses, one tough bird. I should know.

** The guy in the co-pilot's seat happened to be looking at the INS wind read out and we went from a 100 kt plus tail wind to a 90 plus kt head wind within a minute.


By the way, you've been flying for a little over 20 years. Well using my average, you've got one more coming. :p

Just kidding, smooth skies my friend.

[c100driver]

From Boeing FCTM

Turbulent air penetration speeds provide high/low speed margins in severe turbulent air.

However, do not allow the airspeed to decrease and remain below the turbulent air penetration speed because stall/buffet margin is reduced. Maneuver at bank angles below those normally used. Set thrust for penetration speed and avoid large thrust changes. Flap extension in an area of known turbulence should be delayed as long as possible because the airplane can withstand higher gust loads with the flaps up. Normally, no changes to cruise altitude or airspeed are required when encountering moderate turbulence.

Been flying Air Transport jets for 30 years, had only one event that could be classed as "severe turbulence". I was just along for the ride in reality as the only thing we could do was approximate wings level and pitch somewhere on the horizon. VMC

The FCTM talks about stall/buffet margins not structure integrity!

[Intruder]

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?
Since you do not know that there is any difference in control servo response AT ALL, why change your operational procedures and habit patterns? If there is an FCOM or FCTM recommendation to do something else in a particular situation, then do it. Otherwise, rely on proven habit patterns.

[latetonite]

Intruder, that is exactly what I am getting at. But I think you were sending false waves in your previous post.

[Gretchenfrage]

Having gone though a sever turbulence experience as well, i remember seeing every indication between M.57 and M.92. The AP disconnected, just as did the AT, the PFD showed about every tape and color possible. The FD was chasing VNAV and was about as erratic as the other indications.

Now, would V/S 0 had helped me, or for that matter ALT HLD? Not very much better, because the altitude was bound to fluctuate as well, as a consequence of the unstable air.

I longed for the good old turbulence mode in the DC10, which was basically an ATT HLD function (hold pitch and wings level), because this is what saves your a$$ through the worst few seconds.

AF447 showed that chasing speed is fatal and i would pretend that chasing altitude in such real severe situation is similarly dangerous.
Keep the typical nose up, today's big birds normally want between 0 and 3degs up, and keep the wings level, then wait until you exit the worst.

The automatics never really helped me there, they most often just hand back controls when it gets hot!

[Lumps]

As the investigation of Northwest Flight 705 proceeded, other jet transports became involved in similar upsets. These pitch upset events were collectively referred to as "Jet Upsets." This terminology was used because the phenomena appeared to be unique to the new generation of swept wing jet transports which began to enter service a few years earlier. The investigation of Northwest Flight 705, and associated similar pitch upset incidents, led to changes in operating procedures and design requirements for jet transports, as well as improved forecasting and dissemination of hazardous weather information to Air Traffic Control and Flight Crews. These actions proved effective in substantially reducing the occurrence of this type of pitch upset events.

From the FAA website. Anyone know what the design requirements were? What did they get so wrong design-wise on the early jet transports?

[vilas]

A normal line pilot armed with FCOM is not in a position to device procedures for upsets at the extremes of flight envelope. Why not refer it to the manufacturer? They have the equipment, the test pilots, required aerofoil data etc, they are in better position to come up with safe procedures. It is their job.

[Goldenrivett]

What did they get so wrong design-wise on the early jet transports?
B707s & 720s didn't have powered flying controls (apart from rudder boost for take off & Landing). They used control tabs to deflect the control surfaces and had a powerful electric stab trim system. The crew on 705 ran the stab trim full nose down during part of their upset recovery and then stalled the trim motor when attempting to recover from the dive.

"It was believed by the investigators that the essentially simultaneous application of both full elevator and nose down trim placed the airplane in a situation that rapidly became unrecoverable. The pilot was believed to have been attempting to preserve airspeed and altitude in response to the extremely high vertical speed in the updraft, and made the aggressive control inputs to avoid a further degradation of the flight path. The airplane rapidly achieved a vertical, high negative G dive, and upon application of controls in an attempt to recover, the horizontal stabilizer trim motor stalled, and in its full nose down trim position, overcame the elevator capability, preventing dive recovery."

Also Artificial Horizons were poorly designed see "Flight Instruments" towards the end. Weather radar display was green and black and needed skill with the manual controls to work out what was clear air beyond the cells or saturation and no radar image of the storm further away. They were no where near as good as modern colour displays.

See Lessons Learned (http://lessonslearned.faa.gov/ll_main.cfm?TabID=1&LLID=66&LLTypeID=2)

[misd-agin]

Turbulence? Just leave it in VNAV or Profile.

Severe turbulence is a completely different story. Wings level and pitch as necessary to maintain control. If over/under speed is not a significant threat pitch should be approx. 2.5 NU (same as airspeed unreliable for cruise altitudes = OPT ALT).

Some a/c, like the 757 or 777-300, especially if above OPT ALT, will require 3.0 NU.

2.5 NU is a line on the PFD and a good target to shoot for to establish a target pitch attitude.

[latetonite]

If you are able to distinguise between 2.5 and 3 degrees pitch, you are not in severe turbulence.

[Uplinker]

The Airbus 330 has a turbulence damping function:

The purpose of the turbulence damping function is to damp the structural modes induced by atmosphere turbulence. The function uses the Nz accelerometer and two dedicated Ny accelerometers. The PRIMs compute a turbulence damping command, which is added to the normal law command for the elevator and the yaw damper.

I expect the A340 and A380 have it too.