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What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sat Sep 04, 2021 4:06 pm
by convair880mfan
I know that there are some differences in the way that Boeing and McDonnell Douglas aircraft approached how pilots dealt with manually trimming their aircraft and perhaps how they dealt with runaway trim. Don't know how Airbus and other airliners handle these issues. What do you see as the pros and cons of the various designs from a pilot's perspective? [MD suitcase handles versus large manual pitch trim wheels and so on]

Also . . . how difficult is it to fly various airliners with manual trim alone?

Thanks for any information and opinions.

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sat Sep 04, 2021 7:08 pm
by fr8mech
The large manual trim wheels went away with the B727 or the early B737, if I’m not mistaken. On the Boeing’s, you stop a runaway trim by flipping the Stab Trim cutout switches. This kills hydraulic power to the stab motors. Then you trim with the alternate trim, which is an electric trim.

When I get to work, I’ll see what Airbus and Douglas did, but I can’t imagine it’s much different.

Stop the runaway…or uncommanded trim. Then trim using the alternate method. If that doesn’t work…fly the aircraft.

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sat Sep 04, 2021 9:05 pm
by Horstroad
fr8mech wrote:
On the Boeing’s, you stop a runaway trim by flipping the Stab Trim cutout switches. This kills hydraulic power to the stab motors. Then you trim with the alternate trim, which is an electric trim.

When I get to work, I’ll see what Airbus and Douglas did, but I can’t imagine it’s much different.

On the MD-11 both the suitcase handles and the trim switches on the control wheel go to the same primary trim control valves. The suitcase handles are just a mechanical backup in case of an electric or electronic failure. I've never tried to override the solenoids with the suitcase handles, but from looking at the schematic it appears the mechanical controls should be able to override the electronic controls. So in case of a runaway due to a fault in the electronic controls all you can do is move the suitcase handles the other way or split them to stop the uncommanded trim and then open the circuit breaker hoping the manual trim switches caused the runaway and not the automatic trim, as the automatic trim signals come directly from the Flight Control Computers without a circuit breaker or cutout switch in between.

There's no backup in case of a hydraulic failure, so a scenario where you're left with no flight controls but stabilizer trim cannot exist (at least I can't think of one).

This system looks pretty unsafe from just looking at the schematics, with no backup in case of a hydraulic failure and no way to inhibit automatic trim... clearly designed in another time. But still, I've not heard of a DC-10 or MD-11 accident due to stabilizer runaway. Anyone?

I would be interested in a pilots perspective and what the actual procedures are in case of a stabilizer runaway on the MD-11 or DC-10. At the moment all I can do is look at the schematics and system descriptions.

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sat Sep 04, 2021 10:02 pm
by GalaxyFlyer
The B707-B737 had true manual trimming but only as a backup to the normal trim pitch trim controls which were powered. Use of wheels was emergency only, not normal. All more recent airliner types used multiple means of actuating the power to trim the stab—electric or hydraulic motors.

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sat Sep 04, 2021 11:56 pm
by Woodreau
Horstroad wrote:
But still, I've not heard of a DC-10 or MD-11 accident due to stabilizer runaway. Anyone?


First thought to my mind was Delta 1080

But then it was a L1011 and it was a jammed elevator not stab. So that’s not a valid answer

https://www.washingtonpost.com/archive/ ... 3a089bf6d/

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sun Sep 05, 2021 1:03 am
by Starlionblue
On the A330 and A350, trimming is done via trim wheels and trim switches respectively. On the older A330s, you have to manually set takeoff trim after engine start. On the newer A330s and on the A350, the process is automatic.

We never trim manually unless the flight controls degrade to direct law or backup.



Horstroad wrote:
fr8mech wrote:
On the Boeing’s, you stop a runaway trim by flipping the Stab Trim cutout switches. This kills hydraulic power to the stab motors. Then you trim with the alternate trim, which is an electric trim.

When I get to work, I’ll see what Airbus and Douglas did, but I can’t imagine it’s much different.

On the MD-11 both the suitcase handles and the trim switches on the control wheel go to the same primary trim control valves. The suitcase handles are just a mechanical backup in case of an electric or electronic failure. I've never tried to override the solenoids with the suitcase handles, but from looking at the schematic it appears the mechanical controls should be able to override the electronic controls. So in case of a runaway due to a fault in the electronic controls all you can do is move the suitcase handles the other way or split them to stop the uncommanded trim and then open the circuit breaker hoping the manual trim switches caused the runaway and not the automatic trim, as the automatic trim signals come directly from the Flight Control Computers without a circuit breaker or cutout switch in between.

There's no backup in case of a hydraulic failure, so a scenario where you're left with no flight controls but stabilizer trim cannot exist (at least I can't think of one).

This system looks pretty unsafe from just looking at the schematics, with no backup in case of a hydraulic failure and no way to inhibit automatic trim... clearly designed in another time. But still, I've not heard of a DC-10 or MD-11 accident due to stabilizer runaway. Anyone?

I would be interested in a pilots perspective and what the actual procedures are in case of a stabilizer runaway on the MD-11 or DC-10. At the moment all I can do is look at the schematics and system descriptions.


The A330 and A350 have no backup in case of complete hydraulic failure. And there is no procedure for stabiliser runway.

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sun Sep 05, 2021 9:20 am
by Horstroad
Starlionblue wrote:
The A330 and A350 have no backup in case of complete hydraulic failure.

I just looked into the B777 and there's no electric backup either. It's similar to the MD-11. Actually it's almost the same setup except for the hydraulic shutoff controlled by the cutout switches. The alternate trim levers are just a mechanical backup routed to the same stabilizer trim control modules already controlled by the pitch trim switches.

fr8mech wrote:
On the Boeing’s, you stop a runaway trim by flipping the Stab Trim cutout switches. This kills hydraulic power to the stab motors. Then you trim with the alternate trim, which is an electric trim.

What Boeing's do have electric trim? B757/B767?

Re: What are the pros/cons of various airliner types to the situation of runaway trim and manual trimming?

Posted: Sun Sep 05, 2021 9:42 am
by Max Q
If you oppose stab trim motion on the 727 with the yoke you will engage the stab trim brake and all trimming will stop


Good design as that would be your typical first reaction with a trim runaway



At that point you can decide whether to activate the stab trim cut out switches or release the trim brake, of course that decision would be made after completing the relevant checklist