TL position when in reverse

[JammedStab]

It seems to me that on the od JT-8D, that you could not get reverse unless the Thrust lever was closed. And if it was in reverse, the thrust lever was physically held from being moved forward.

Reading this incident report about an A300 that ended up having an engine in reverse after a go-around, it sounds like the thrust lever can be moved forward when reverse has been selected but the engines computer will keep it at idle.

Is this how it is now for all the modern engines such as CFM, CF6, Trent, etc?

Thanks.

[tdracer]

I don't know about Airbus, but on Boeing aircraft there is linkage as part of the throttle lever that:
a) prevents movement of the reverse lever if the forward lever isn't at idle, and
b) prevents movement of the forward lever from idle if the reverse lever isn't at idle.
It's all independent of the engine type - for example the throttle levers are common for all 747-400s/-8 regardless of the engine type.


It would take a physical breakage of that linkage to allow movement of one lever without the corresponding lever at idle.

[Oakape]

As with the AP/FD system, what you have selected is not necessarily what you actually have. With the reverse thrust levers in the stowed position, the thrust levers will be free to be moved out of the idle position. However, if there is a malfunction, the engine itself may still be in reverse. If I remember rightly, after the Lauda crash modifications were made to Boeing aircraft so that the engine is commanded to idle if the reverser sleeves on the engine are sensed to be out of the stowed position & the aircraft is not on the ground.

[DevX]

I thought that 'squat' switches were now used on the undercarriage which prevents reverse thrust being operative whilst airborne, irrespective of where the levers are.

[tdracer]

If I remember rightly, after the Lauda crash modifications were made to Boeing aircraft so that the engine is commanded to idle if the reverser sleeves on the engine are sensed to be out of the stowed position & the aircraft is not on the ground.

Oakape, there has always been a feature that will command the engine to idle if the T/R is not in the commanded position (it's required by the FARs). On the older engines it was done with a mechanical feedback linkage, on FADEC it's all done with electrons.
Post Lauda, the T/Rs were retrofit with a 'third lock' - a lock on the synchronizing shaft (aka "sync lock") that would prevent reverser movement. The newer aircraft (787, 747-8) have a similar lock but it's on the T/R guide track (aka "track lock").

[tdracer]

DevX - again, speaking Boeing here, but aside from some of the very early jet aircraft (which had flight operable T/R - I'm thinking maybe the DC-8 had those), there was always 'air/ground' to enable the T/Rs on the ground. Unfortunately that could also cause problems: A 737-200 crashed in Canada in the mid 1970s - they landed then discovered a snowplow on the runway after they deployed the reversers. The crew closed the reverse levers attempted to takeoff again, but one of the reversers hadn't completed the stow cycle when air/ground went air which closed the T/R hydraulic isolation valve. Without the reverser locked in forward, the aero forces opened the reverser again (those big clamshell type) and they crashed.

[flyingchanges]

And now we know just where the Boeing warning of "must not go around after reverser deployment" comes from.

[Turbine D]

Original quote by tdracer DevX - again, speaking Boeing here, but aside from some of the very early jet aircraft (which had flight operable T/R - I'm thinking maybe the DC-8 had those)

Indeed, the original DC-8s were capable of reversing thrust in the air, but only on the two inboard engines, the CFM56 re-engined aircraft did not. It was used for rapid descent. Some of the pilots would announce ahead of time they were going to use in-flight thrust reversing as the resulting vibrations in the cabin were extremely noticeable. Interestingly, I read the Concorde also had this capability as did the HS Trident.

[tdracer]

flyingchanges - starting with the 757/767 (which was in initial design at the time of the 737 crash), the T/R is designed for this scenario. Depending on the model, all current Boeing aircraft have one of two basic systems that will keep the isolation valve open if the reverser isn't stowed.


Unfortunately, it was a malfunction of that 'restow' system that lead to the Lauda deployment (the third lock system that was added after Lauda is to prevent a recurrence of that scenario).

[flyingchanges]

Yes, but the legal team always prevails...

[JeroenC]

The 738 can deploy below 10 ft RA. How i know? Well, its also in the books😂 But why can they be? Mr B explicitly forbids it.

[glendalegoon]

regarding the radio altitude and thrust reversers.

I seem to recall that a 737 landed and could not get into reverse as the plane "THOUGHT" it was still in the air. I think it was based on compression o f the nose strut (going from memory, excuse me if I am so wrong).

SO a smart guy figured out that if you were at 10' radio altitude for five seconds (or is it 10) you might actually just have a problem with the weight on wheels switch so this radio alt feature would allow you to use the thrust reversers.

It doesn't mean you SHOULD put the reversers out in the air, but just in case you are falling off a cliff and the WOW didn't work, you have a chance.

NOW the DC9 would let you put the thrust reversers out in the air, though you were NOT TO DO SO. SO the only safety aspect was the pilot not putting them out in the air.

I respected douglas so much as they actually tested their planes with thrust reversers deployed to see if you could still fly them. Boeing relied upon theoretical calculations.

[lomapaseo]

DevX

I thought that 'squat' switches were now used on the undercarriage which prevents reverse thrust being operative whilst airborne, irrespective of where the levers are.

If only they all work 100% of the time.

If one depends on a switch and circuit that is quietly not doing it's job every now and then. Then you better have a backup system that does it's job more reliably.

And if you never test these circuits or devices before each flight just what are the combine probabilities that two or more will fail at the same time?

Lauda is just one of many incidents that pointed at unknown probabilities combining together, thus the 3rd lock is now mandatory.(and even that is not 100%)

[tdracer]

I respected douglas so much as they actually tested their planes with thrust reversers deployed to see if you could still fly them. Boeing relied upon theoretical calculations.

Not true, Boeing did flight tests with a T/R deployed as well, including on the 767 (IIRC it was a regulatory requirement to flight test it). But they did the tests at relatively benign conditions and the engine already at idle when the T/R was deployed, not at 24k/Mach .78 and the engine at max climb (i.e. Lauda). Even though the FADEC retards the thrust to idle, it comes down the normal decel schedule which - at those conditions - takes over 10 seconds to get to idle. With a T/R deployed on an engine at power, the reverser efflux basically wipes out the lift on that wing and it drops like a rock. By the time the engine got close to idle they'd already lost control of the airplane.


BTW you are correct about the 737 - it has a tendency to 'float' enough that the squat switch WOW wouldn't go true - so they use radio altimeter.


Newer Boeings, such as the 747-8 and 787 use a combination of both radio altimeter and squat switch (I don't recall off hand what we did for the 777). At least one radio altimeter (less than 5 ft.) and one squat switch need to indicate ground before you can deploy the reversers.


Lomapaseo - NO system is 100% reliable. The certification standard for flight critical systems is less than one catastrophic failure per billion flight hours (10-9/hr.). Since Lauda, Boeing T/R systems are among the most scrutinized systems on the airplane - and even taking into account potential latent faults, the probability of an inflight deployment is ~1 per trillion flight hours (10-12/hr.).

[Oakape]

Thanks for your posts tdracer, they make the whole issue a lot clearer.

[Denti]

@lomapaseo:

dunno about other planes, on the 737 though there are four ground/air sensors on the main landing gear and at least one on the nose gear and they are monitored by the PSEU system on the ground. If one fails you will get a non-dispatchable fault. Had that happen a few times lately with sensors that were just out of range, one was quite funny as it caused the PSEU warning to come on in left turns and switched it off again in right turns.

And if all those sensors fail i still can open reverse if i want to, after all below 10ft RA it is possible. It is not a recommended procedure in normal flight operations of course.

[JammedStab]

To quote from the report...

"After both throttle control levers were advanced to the takeoff position, the No 1 engine thrust reverser locked but the No 2 engine thrust reverser remained unlocked for the rest of the flight. Engine thrust increased on the No 1 engine but the No 2 engine remained at idle thrust."


"During the deploy command, the FADEC software logic restricts the fuel flow to approach idle fuel flow until a signal indicating 78% of full deployment is received, regardless of throttle control lever position."

http://www.aaib.gov.uk/cms_resources...LK%2005-12.pdf

So it appears that with the FADEC systems, the thrust lever can be physically moved forward of Idle while in reverse.

[tdracer]

So it appears that with the FADEC systems, the thrust lever can be physically moved forward of Idle while in reverse.

Again, not true on Boeing - I can't comment on the A300. Perhaps Airbus does this differently as well

[tdracer]

Jammedstab, after further consideration, I think I answered a question you didn't ask....


Pre-FADEC, if there was an uncommanded T/R deployment, the mechanical linkage that restricted thrust to idle would have had the affect of holding the throttle at the idle stop. Basically, since it was a mechanical system, that's how it had to work. We called it the "throttle snatcher" since it would physically grab the throttle cables and move them to idle - including the thrust lever.


With FADEC, that's all done with electrons - so there is no need for the throttle snatcher. If there is an uncommanded T/R deployment, there would be nothing that prevent 'normal' throttle movement in forward. It just wouldn't do anything since the FADEC would be commanding idle.

[JammedStab]

Thanks for the info.

Regarding the 737 crash you mentioned earlier, I believe the report stated that they were able to figure out from the autopsy that the thrust lever moving aft broke the captains thumb. This is the snatch you mentioned.

Separately, I remember on the 727, with the engines shut down, we would sometimes check the interlock. With the thrust levers(TL's) fully aft, the reverse levers(RL's) could be selected. When the TL's were moved only slightly forward from the closed position, the RL's were mechanically blocked from being selected. Likewise, if the RL's had been selected, the TL's could not be moved forward from the fully aft position. I suppose that is different than an engine staying in reverse by itself after deselection as happened in this incident.

I believe that this is all mechanically located in the Thrust Lever unit.

I am hesitant to just go ahead and try this on the CF-6 engine I am operating these days. Has anybody else tried.