Selecting VS zero during turbulence in cruise
Join Date: Dec 2005
Location: the Great Southern Land
Posts: 67
Likes: 0
Received 0 Likes
on
0 Posts
Flight Path Angle?
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 ?
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 ?
Aviator Extraordinaire
Join Date: May 2000
Location: Oklahoma City, Oklahoma USA
Age: 76
Posts: 2,394
Likes: 0
Received 0 Likes
on
0 Posts
It was always drummed in to me that during turbulence one should attempt to hold an attitude, not an altitude
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.
Also, it is my experience that a very few number of pilots have ever experienced any extended period/s of real/actual extreme turbulence.
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...
Aviator Extraordinaire
Join Date: May 2000
Location: Oklahoma City, Oklahoma USA
Age: 76
Posts: 2,394
Likes: 0
Received 0 Likes
on
0 Posts
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.
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 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.
Just kidding, smooth skies my friend.
From Boeing FCTM
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!
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.
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.
The FCTM talks about stall/buffet margins not structure integrity!
Last edited by c100driver; 9th May 2014 at 21:12. Reason: for clarity
Join Date: May 2000
Location: Seattle
Posts: 3,196
Likes: 0
Received 0 Likes
on
0 Posts
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?
Join Date: May 2005
Location: middle of nowhere
Posts: 312
Likes: 0
Received 0 Likes
on
0 Posts
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$$ through the worst few seconds.
AF447 showed that chasing speed is fatal and i would pretend that chasing altitude in such real severe situation is similarly dangerous.
Keep the typical nose up, today's big birds normally want between 0 and 3degs up, and keep the wings level, then wait until you exit the worst.
The automatics never really helped me there, they most often just hand back controls when it gets hot!
Now, would V/S 0 had helped me, or for that matter ALT HLD? Not very much better, because the altitude was bound to fluctuate as well, as a consequence of the unstable air.
I longed for the good old turbulence mode in the DC10, which was basically an ATT HLD function (hold pitch and wings level), because this is what saves your a$$ 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!
Join Date: Jun 2010
Location: TinselTown
Age: 45
Posts: 203
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jun 2007
Location: Wanderlust
Posts: 3,407
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 720
Likes: 0
Received 0 Likes
on
0 Posts
What did they get so wrong design-wise on the early jet transports?
"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
Join Date: Jan 2006
Location: US
Posts: 2,205
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
The Airbus 330 has a turbulence damping function:
I expect the A340 and A380 have it too.
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.