Turb switch

[Cavallier]

Out of curiosity what was the actual function of the Turb switch on a Classic?? Also was it a mode that was used very often????

The Cav

[rudderrudderrat]

Hi The Cav,

I don't know about the Classic, but the L1011 had an autopilot TURB mode (PFM). It worked really well and prevented large pitch excursions whilst chasing small deviations in altitude.

Pity AF447 didn't have one fitted.

"Turbulence mode provides pilot-assisted, constant attitude flight through turbulent air. The autopilot control authority is reduced in order to soften its response to attitude changes caused by the turbulence.

Turbulence is an autopilot mode only and cannot be selected unless an autopilot is engaged. The TURB mode may be selected any time either autopilot is in CWS or CMD and A/L TRACK or GO AROUND is not established. The TURB light will illuminated indicating mode selection. Verification of mode engagement is accomplished by checking for TURB legend on in both AFCS MODES annunciators. When turbulence is selected while the autopilot engage lever is in the CMD position, the solenoid held lever drops from CMD to CWS. This action essentially cancels all other modes.

No other mode can be selected while the turbulence mode is engaged. The turbulence mode can only be released by pressing the TURB switch a second time or by disengaging the autopilot. Either of these actions then, is a prerequisite to resuming normal autopilot-controlled flight after a turbulence encounter.

If a flight director is also on when turbulence is selected, its command bars will be biased out of view but no FD warning flag will appear. When TURB is canceled, the command bars will return if the FD switch was left on but it will have a new reference. The new reference is the attitude existing at the time of TURB cancellation since all previous modes were canceled when TURB was selected."

[oxenos]

It was used if the cabin crew told us that the passengers would not sit down when the seat belt signs were switched on. Selecting it produced computor generated turbulence through the auto-pilot, which got progressively more pronounced until they were all strapped in, when it could be switched off.

[Meikleour]

CAV: If I remember correctly, it continued the nav/heading part of the autopilot but the pitch had to be controlled through the pitch wheels. ie. the A/P would not try to hold the altitude - this was done with the pitch wheels.

[The Grim EPR]

There is a little more info at the b737.org site

Automatics - An illustrated guide to the different MCP's & autopilots used on the 737 (http://www.b737.org.uk/glareshield.htm)

[40KTSOFFOG]

If memory serves me correctly it was also a feature of the A300-B4 autopilot as well. Only every used it once!

[Zorin_75]

Pity AF447 didn't have one fitted.
How could it have helped them?

[rudderrudderrat]

Hi Zorin_75,
How could it have helped them?
If they had a "TURB" mode, it would have maintained wings level and an attitude they selected. They then could let go of the stick and simply set the power.

It was a very basic autopilot mode which only required attitude information.

[Piltdown Man]

The TURB Switch has another name. In every aircraft I've ever flown it's called the Seat Belts sign. Turn them off and the turbulence starts.

PM

[Zorin_75]

If they had a "TURB" mode, it would have maintained wings level and an attitude they selected. They then could let go of the stick and simply set the power.

It was a very basic autopilot mode which only required attitude information.
You mean as a simpler mode the a/p could revert to instead of disconnecting? Makes sense. Wonder what attitude they'd have selected, though...

[M2dude]

Concorde had a TURB autopilot mode also. I can never remember it being used much, if at all. Probably similar to the Classic/L1011 concept, the idea was that the aircraft maintained pitch and heading hold (the basic engage modes) but with reduced gain to prevent large axis excursions.

Best regards
Dude :O

[Denti]

Guess it was not a feature of the classic, it was a feature of the so-called "jurassic" models 100 and 200 (incl 200ADV).

[Cavallier]

Thanks guys. in laymans terms it was basically a wing leveller!!! I would very much be interested in any stories of when (on the rare occasion) you used it.

The Cav:cool:

[KBPsen]

You still haven't told us which aircraft type you are asking about.

[Slasher]

TURB mode on the 747-200/300 (from memory) was a basic
CWS, and flight control surfaces deflected by half the normal
deflection for a given stick input.