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View Full Version : UNSCHD STAB TRIM on T/O: Go/No Go ? (B 757/200)


deepakt
7th Aug 2001, 07:34
In July at around mid day (45 deg c approx.) T/O was rejected at 140 kts, in Dubai, when UNSCHD STAB TRIM EICAS message was displayed. Result three burst tyres, molten brakes. B 757 handles well without stab trim. Would you reject? :confused:

HPSOV
7th Aug 2001, 18:02
Above 80 kts reject for engine failure, fire, or if aircraft unfit for flight.
Its gotta be a pretty tough call on the day as to whether an unscheduled stab trim would cause an aircraft to be unfit for flight.
On one hand I think no, just flip the cutout switches and sort it out once you're stable in the air.
On the other hand, what if it put in full nose up during rotate? Would you react quick enough to prevent a stall. What about full nose down causing you to drive off the end of the runway.
I guess at the end of the day its going to come down to the particular captain.
And in the case you present no one died, and the aircraft was re-useable, so you cant question his actions too much.

Eff Oh
7th Aug 2001, 18:27
Personally....I would think so, yes. If in doubt as to the effects, stop! I have never flown the B757 with an unsched stab trim on rotate.......Has anyone?? Even if you hit the cut out switches, the trim is no longer in the correct position for takeoff. Tailstrike, stall, or off runway excurstion all worse than a few burst tyres! My opinion only though! :)
Eff Oh.

Bally Heck
7th Aug 2001, 21:52
The 757 has no drill for a jammed stabiliser because Boeing reckon that it can be flown out of trim. Tried it in the sim and it's hard work but managable.

The inference from that is that even with the stab at it's limits, the elevator could overpower the out of trim forces of the stab. :confused:

If this is the case, (and I'm not saying that it necessarily is) (but it might be) then it should not present a major problem getting airborne if you have big muscles like me :p

Roadrunner
8th Aug 2001, 01:15
Yes sir, I would have stopped, presuming I felt I could do so safely.

For that matter, I’d stop for an engine overheat above 80 kts as well. Why take something like that into the air unnecessarily with adequate runway remaining.

The book. A guide for wise men and a bible for fools.
:o :o

machcruiser
8th Aug 2001, 17:00
After 80 kts its consider high speed. Rejecting for advisiory or caution messages are not advisable during hight speed phase of the takeoff roll. Thats why Boeing has inhibited the caution lights after 80 kts. :)

HOMER SIMPSONS LOVECHILD
9th Aug 2001, 17:26
It's your decision on the day.Thats what we earn those popstar wages for!
Please stick to your brief though.You tell the other guy that you will only abort above 80 for a very small list of things,one of which is some variation of the vague"Inability of the aircraft to sustain controlled flight".To my mind that is a triple hydraulic failure,Cessna wrapped round the tailplane,half the wing on the bumper of a land rover type scene.It will take a bit of work but you can fly out with an out of trim stab.Its the old "Go Mindedness" argument.Statistics prove its the way to go but we all know about statistics dont we.

Roadtrip
9th Aug 2001, 19:47
The book says the 757 can be flown without stab trim. The 767, not necessarily so in all conditions, hence the pitch enhancement PTU from the right system. (that's assuming the trim itself is operational).

quid
11th Aug 2001, 07:46
I think many of us in the flying business have taken this "High speed aborts are Dangerous" to the extreme.

Sure, you can be at Max TOW for any given runway, but there are thousands of departures every day at weights WELL below that. The acceleration of most transport jets on takeoff roll are very similar. (Excluding the extreme density altitude and very heavy weight/high temperature takeoffs.)

Most jets get to 80 kts. only about 2000' into the takeoff roll, and about 110-120 at the 3000' point. Now, if I'm on a 10,000 foot runway, and I abort for any reason at 80 kts., I've got 8000 feet in which to get it stopped! From 115 kts, I've got 7000'. What's the problem?

No way I'm taking a fire warning or other questionable situation in the air (on purpose) when I've got all that runway left.

Sure, there are situations where you ARE right up against the MTOW. AT real heavy weights, your V1 might be 150 or so. We get paid to make plans in the event something out of the ordinary occurs.

When you research the accident reports involving high speed aborts/attempts, you will find that many of those happened AFTER V1, and many of those involved confusion in the cockpit, one guy pulling, the other pushing.

In my opinion, it's not the high speeds that are the factor, it's the lack of pre-planning for THIS takeoff.

Every takeoff is different. I urge you not to get into a simplistic mindset. Would your decisions be any different if the two runways were 6000' vs 12,000? Mine sure are.

:)

machcruiser
11th Aug 2001, 12:38
While going through books on this unschedule stab trim came across a review & study guide of an european airline which states if " you observe the yoke moving towards you and you recognize a runaway stab trim you grab the yoke. Now do you push or pull ? You must help the autopilot to overcome the pressure introduced by the trim motor. So if the runway stab is trimming forward (nose down) and the yoke is moving backwards you must PULL! if the yoke is moving forward you must push."
As far as I know you are suppose to apply opposite pressure and then cut off the stab trim cutoff switches. Could somebody explain the above? :(

ironbutt57
14th Aug 2001, 01:18
common sense prevailed...nobody got hurt...plenty of room in dubai....not at chicago midway... :) :) :)

quid
14th Aug 2001, 06:57
machcruiser-

We have to explore two different regimes of flight.

First, on the ground during takeoff roll. No autopilot (that I know of on big jets) will trim the elevator at all. It will trim the stab, and the yoke doesn't move. So a runaway stab on t/o won't/can't move the yoke. If the yoke moves by itself, it's something else.

Now, in flight with the autopilot engaged in altitude hold, if the trim starts running away nose down, the yoke will initially move back to hold altitude with elevator, until it reaches the force limit, at which point it will disengage. You will then have to continue to hold back pressure to hold altitude.

charliecossie
15th Aug 2001, 03:45
What I'd like to know is when are Royal Bloody Nepal actually gonna fix the damned thing? (three weeks and counting)
Hope you manage to do it quicker than the spoilers......
:mad:

Bally Heck
15th Aug 2001, 15:28
Just read my Boeing FCTM and under Uscheduled Stabiliser Trim it says "Use opposite control column force and main electic trim to maintain the desired pitch angle. This will most probably resolve any mistrim due to runaway" (my italics)

So according to the bible he would most probably have got away with continuing the take-off. Bit worrying that. :eek:

Roadtrip
15th Aug 2001, 21:22
The immediate actions for unscheduled stab trim is "Stab trim cutout switches - Cutout." Fooling around with using opposite direction stick to stall/cutout the trim motors is a bit crazy on takeoff roll, rotate, or initial climbout. I'd be very inclined to abort the takeoff. Taking an airplane into the air with maybe severe and worsening control problems makes the risk of damage due to high speed abort less onerous.

machcruiser
16th Aug 2001, 06:13
Quid.

Thanks for the explanation. Does clearup things now. :)