Uncommanded thrust reverser deployment in flight [Archive]

sheppey

4th Sep 2017, 13:21

For the B737 Classics, the thrust lever for that engine will close to idle immediately without pilot action. This takes place so that only idle reverse is experienced and is easily countered. There will be some buffet. Follow the QRH once you have the aircraft under control. If roll is severe due to high power on the good engine, immediate closing of the thrust lever on the "good" engine should minimize the severity of the roll
I understand in the B737 NG series the engine also automatically goes to idle reverse but the thrust lever doesn't move until you go through the QRH and take the published action.
I recommend you Google the report (see link) on the Lauda Air Boeing 767 that experienced an uncommanded in flight thrust reverser during climb. On that accident the aircraft rolled rapidly and control was lost. I recall reading that investigation showed the crew had four seconds to take corrective action before loss of control would occur. That corrective action was not specified but I presume would have included immediate closing of both thrust levers and knowledge of unusual attitude recovery on instruments.
See: https://en.wikipedia.org/wiki/Lauda_Air_Flight_004

How could you practice for a catastrophic event in a simulator?

Depending on the type of simulator, the uncommanded in-flight thrust reversal should be selectable on the simulator instructor station

Goldenrivett

4th Sep 2017, 13:22

I still wonder how the average crew would handle this situation if it happened to them.
It happened to a Lauda Air crew on B767 in 1991.
Lauda Air B767 Accident Report (http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/LaudaAir/LaudaRPT.html)
Since then, we have always had a QRH / ECAM procedure.

Basically it says to reduce thrust (Idle), Reduce Airspeed, Shut the engine down, control the aircraft with full rudder trim and sufficient bank to maintain heading.

t7a

4th Sep 2017, 13:32

Also happened to a Fk100 (Brazil I think). Affected side thrust lever automatically slams closed (will take your fingers off if in the way). Believe the crew were unaware of this function with disastrous consequences.

paradoxbox

4th Sep 2017, 13:39

So do all boeing and airbus aircraft have reliable indicators that the reverser is indeed the problem?

The reason I asked this question is because the problem-solving you have to do is nightmarish if you don't have any clear cockpit indicators of what is happening. The big trouble is if you or the other guy shuts off the engine and that turns out not to have been what was causing the problem. I suppose that is why the checklists say to return the suspect engine to idle rather than shut it down, as I have seen some people do in incident reports.

Imagine you've just taken off, a little bit beyond V2 maybe, flaps start coming up, then BAM you experience severe yaw or roll and your airspeed starts decreasing. Is a thrust reverser the first thing that comes to your mind? What other possibilities are there that could cause this state? Severe flap asymmetry and/or uncommanded spoiler deployment?

Uplinker

4th Sep 2017, 14:11

Airbus FBW have a boxed "REV" indication that appears on the respective N1 or EPR gauge - amber while reverser is travelling, green when in correct position.

The FADEC will command idle thrust in the event of one or more reverser doors unlocked in flight or not in commanded position, and/or will command auto restow.

You might also get a ENG 1(2) at IDLE message and ENG 1(2) SHUTDOWN command on ECAM. It tells you to set thrust to idle and engine master off, along with speed limit, max rudder trim, and LAND ASAP amber.

.

paradoxbox

4th Sep 2017, 14:16

those appear when you command reverse thrust, but i think the most common situation this is going to happen is when maintenance does sloppy work and something goes awry. in this situation, are your indicators still going to show up? and if you've got no indication, are you going to be able to diagnose what the problem is without that indicator?

as you say, the loss of control and utter lack of time you have to solve the situation before it becomes unfixable is a little unnerving.

vilas

4th Sep 2017, 14:26

In airbus it should not be a problem. There is case from Safety first below, the aircraft was despatched with one reverser deactivated:
the aircraft was dispatched with the thrust reverser unlocked and free to move under aerodynamic forces.
Less than 4 minutes after take-off, the engine N°1 auto-idle was activated. It activates once there is more than 10% opening of the reverser, and brings down the TRA to idle based on the initial TRA position. This reverser opening detection also triggered the Auto thrust disconnection. The Auto re-stow which is also normally triggered was not effective since there was no hydraulic power due to the proper de-activation of the HCU.
As described in the Flight data analysis, there was very little changes on aircraft flight characteristics. Based on the flight parameters evolution it is assumed that the thrust reverser deployed slowly due to the aerodynamic forces (there was no hydraulic power due to HCU de-activation).
The aircraft maintained control with no upset throughout the event.
The engine was shut-down, then the Auto thrust was re-engage, and an IFTB was made followed by an uneventful single engine landing.

lomapaseo

4th Sep 2017, 14:27

The uncommanded reverser problem that most talk about is Lauda (there are others)

The pinch point in control of the aircraft for some aircraft is the climb portion where the wing control surfaces are defeated by the reverser eflux.

Yes there are some things that might recover but unless you can act real fast in the right direction ..... I know of one where the PF just continued the aircraft roll through 360 deg and completed the flight without his passengers knowing it happened.

The best corrective action is to act as soon as the reverser unlock light flickers and reduce the engine to idle (no need to shut it down in most cases).

If the discussion is to be about business jets alone, then my comments above may not apply.

tdracer

4th Sep 2017, 14:48

On a twin with most large turbofan engines, if the reverser deploys in flight your chances of a happy outcome are pretty minimal. I was involved in the Lauda investigation - including wind tunnel testing with a reverser deployed. It's not just the thrust asymmetry - the big problem is that the reverser efflux effectively spoils the lift on the wing and that side drops like a rock.
As a result, the cert basis for all Boeing aircraft has changed from 'it's controllable' to 'it'll never happen (3 independent failures, probability less than 10E-10 per flight hour).
It's a regulation that, with the reverser in an uncommanded position, the engine can not provide more than idle thrust. On the pre-FADEC engines this was done mechanically with a fairly complex feedback mechanism that would move the throttle to idle (rather forcefully as sheppey notes - yes it'll break your hand if it's in the wrong place). With FADEC it's all done with electronics and the throttle doesn't move but the engine will still be commanded to idle. The main problem - as Lauda demonstrated - is the transient. When Lauda deployed, the engine was at max climb and the decel characteristics at 24k/Mach .78 meant the aircraft was long since out of control before the engine reached idle.
Paradoxbox, Boeing displays an amber/green "REV" above the EPR/N1 display to indicate the reverser is in-transit (amber) or deployed (green) - and it's a robust indication. IF the reverser has in fact deployed it's not likely to help much.

galaxy flyer

4th Sep 2017, 16:13

The C-5 Ramstein crash was attributed to a TR deployment near lift-off. Yes, the destruction of lift by the TR efflux is probably more a factor than just the TR. On aft-mounted engines, the effect is drag caused by the TR panels and reversed thrust without the lift destruction component. Still the highest yaw of all the engine failure modes and add in a crosswind, control becomes difficult. IAS is usually limited at about 200 KIAS, depending on type.

IcePack

4th Sep 2017, 17:18

Back in the day, a particular nasty trainer gave me a thrust reverser deploy (on the sim) at full power at rotate. (Eng remained at full power) Don't think it was even possible on a 737 but my call of feather it (just come of turboprops) ensured it was shut down by the capt. we were 30 deg off heading but back on track by 200 ft. My point being sometimes s** the qrh just do whatsynecessary.(remember full rudder & full aileron)

Tu.114

4th Sep 2017, 17:32

During my time on the F70/100, we were sometimes presented this malfunction by the instructor. As has been mentioned by T7A, there is a cable connecting the reverser buckets to the (IIRC) FCU, and on to the power levers, slamming the affected engines lever to the idle position when the buckets open. Although the engine spooled down to a corresponding thrust setting immediately, it was quite a ride and required some work with the controls to keep the aircraft pointed in a good general direction. As soon as the engine was shut down, though, it did not feel much different from an ordinary engine failure except for the affected power lever still remaining stuck in the flight idle position.

I am not sorry I never had to try this in the real aircraft.

EMIT

4th Sep 2017, 17:38

If I recall correctly, the Lauda 767 did have a number of issues with its reverser on previous flights, indicated by (intermittent?) REV ISLN (reverse isolation) light.
Also on the accident flight, the REV ISLN light illuminated intermittently.

Position of reverser is properly indicated on engine instruments, probably on every aircraft type, by "no indication" for stowed, amber "REV" for in transit, green "REV" for deployed. In case of uncommanded deployment, warning display may trigger an alert (depending on aircraft type).

For chance of recovery - at low altitude, low speed, high thrust, chances are dim, lack of maneuvering room. At high altitude, improper reaction may come into play: unfamiliarity with unusual attitude recovery (civilian flight training conditions people to maintain positive gee and to pull to get back up to altitude; in case of severe roll this will lead to a pull through the vertical down, which will invariably destroy a jetliner. Proper reaction would be to unload, roll back or roll through, but keep the nose from dipping down too deep).

Many informative postings prior to this one, compliments.

paradoxbox

4th Sep 2017, 17:51

This is turning into a great thread, thanks for the replies everyone.

But I hope someone will answer my question (perhaps it was not clear enough)

I know that under normal circumstances most aircraft (all?) have indicators for thrust reverser deployment -when on the ground-.

But.. When you are in the air, and the reverser has deployed, for example by aerodynamic forces, or mechanical failure (parts falling off etc) - will the indicators still appear in your aircraft? Is there some kind of mechanical or electrical sensor that physically or electronically detects that the reverser door or doors are not stowed completely? Does it detect when the door is only open slightly or do the doors need to open to the stops before the sensor detects it? If the cause of the deployment is due to faulty maintenance, will the sensor (i.e. in the Airbus) still be able to detect that the door is open?

Sounds like some aircraft have a physical link to the throttles which slams the throttle lever down. That sounds like a nasty hospital bill but certainly better than crashing. What about in the Boeings and others i.e. CRJ?

And if for some reason you did not get a cockpit indicator, do you think you would still come to the conclusion that your reverser was stuck open and act appropriately? Or would you be hunting for other possible solutions and miss the obvious as the Lear crew did?

bafanguy

4th Sep 2017, 18:00

My point being sometimes s** the qrh just do whatsynecessary.(remember full rudder & full aileron)

Yep...

Too long ago for me to remember the date (or apparently for Google either), Eastern Airlines had a TR on a DC9(-30 I think) come open right at rotation or immediately after in KATL.

They were dispatched with it inop but MTC didn't properly secure it so it came unlocked and fully deployed while at T/O power. IIRC, they immediately closed the throttle and shut off the fuel and lived to fly another day. If they had any advantage, it was knowing that TR had been messed with but mostly it was superior airmanship.

IIRC, the reverse component for the -30 was 40% of engine being thrust produced.

We tried the event in the sim during recurrent and if you didn't IMMEDIATELY snap the throttle to idle and shut off the fuel…you didn't make it.

As you said, no time for any book or even thinking about any book.

As for indications, I never flew a TJ airplane where the reverser indications were different air vs ground but I can't speak for all airplane types.

RAT 5

4th Sep 2017, 19:49

We tried the event in the sim during recurrent and if you didn't IMMEDIATELY snap the throttle to idle and shut off the fuel…you didn't make it.
As you said, no time for any book or even thinking about any book.

This says a great deal about today's training in some airlines. Glad to hear there is some imagination in some airlines. I was so tired and fed up of the repetitive trivia served up in LPC's & OPC's. It was mind numbing and not very educational after the umpteenth time of a mandatory item. RST's tried to inject some variances, but the emphasis was on CRM, QRH, FDM & SOP's. There was not a lot of thinking outside the envelope. Guys are terrified of having to solve a problem without a checklist: terrified of acting immediately, as the scenario dictates, as a pilot. Their first thought is "what does the SOP say?" Then there is a huge ? mark as they realise there isn't one. OMG, what to do.
I was in RST's given by 2 year F/O's SFI. They had the script. It was quite interesting to remember the times they commented, "I haven't seen anyone do that before, but it worked. Hm?" That was from the enlightened ones. The others just chimed in, "you weren't supposed to do that." Oh dear.
I used to really enjoy the education gained by designing scenarios where there was no checklist; and often derived from real events that went wrong. Sadly, that was stamped out by those who thought it taught a bad attitude.
How many accidents have happened because the crew failed to follow the QRH correctly? Not many. It is the WTF factor that fuddles the brain and needs a knowledge of the a/c & aviation to solve the problem that trips up the majority. The NTSB investigators are sometimes quite bemused as to 'why the heck did they do that'. Often it was because they didn't know any better, but they should have.

In my some of my previous outfits they had one OPC/LPC program and one RST. You could ask your mates what happened and what the sequence was and what they did. Not very educational or confidence building. RST's should be real training and fun. Find out about yourself & the a/c and the way you behave under certain conditions. Checking should be more realistic and less repetitive trained monkey stuff. Ah, but then too many might fail and need re-training and that is disruptive to the roster, especially in the summer. Tick the boxes and smile. XAA happy, CFO happy. HOT? that depends.

Corrosion

4th Sep 2017, 20:49

On Boeing 717, BR715 engines, there is mechanical connection from reverser scoop structure to LVDT via cable. Cannot recall indication as i am not fully certified engineer for that a/c type, might be transit/deployed light on EFIS. Another proxy sensors (unlock?) are located on lock actuators and senses if locking tongues moves out from locked position. This aircraft have full scoop reversers, like old DC/MD-series.
These reversers are locked mechanically by lock tongues, two/scoop, and overcentering links on pivot point. Can´t remember anymore is hydraulic rams pressurized to stowe all the time when commanded stowe.
Incidents happens for this a/c when reverser deploys during flight, manufacturer made some modifications to overcome this problem.

Like all a/c systems, these gadgets needs lots of maintenance.

lomapaseo

4th Sep 2017, 23:32

Like all a/c systems, these gadgets needs lots of maintenance.

The trouble is when you need lot's of maintenance it becomes a problem by itself to bite you.

Some fail-safe features have their own hidden faults and since they never get used in most of the life of an aircraft system, you don't know they are at fault or won't work when sorely needed.

I have seen too many assumptions in reliability of detection or prevention devices that were never validated during manufacture or installation.

Like a "jezzuz bolt we need balances in our attempts to minimize since we can't seem to prevent anything for sure.

tdracer

5th Sep 2017, 00:36

But.. When you are in the air, and the reverser has deployed, for example by aerodynamic forces, or mechanical failure (parts falling off etc) - will the indicators still appear in your aircraft? Is there some kind of mechanical or electrical sensor that physically or electronically detects that the reverser door or doors are not stowed completely? Does it detect when the door is only open slightly or do the doors need to open to the stops before the sensor detects it? If the cause of the deployment is due to faulty maintenance, will the sensor (i.e. in the Airbus) still be able to detect that the door is open?

I'm only speaking for Boeing here (although I'd be surprised if they other guys are meaningfully different). The T/R translating sleeve position is monitored electronically - prox switches, L(R)VDT, or a combination there of, and works exactly the same for an uncommanded deployment in-flight as it does on the ground (one of the last words on the Lauda flight recorder was "it's deployed" - not that the knowledge helped them much).

Sounds like some aircraft have a physical link to the throttles which slams the throttle lever down. That sounds like a nasty hospital bill but certainly better than crashing. What about in the Boeings and others i.e. CRJ?
Again, speaking strictly Boeing here but would be surprised if the others are meaningfully different. There is a mechanical linkage to something called the 'strut drum box' - the position feedback to this box serves a dual purpose by preventing advancement of the throttle above idle while the T/R is in transit and moving the throttle to idle if the T/R isn't in the commanded position. FADEC eliminated the need by doing everything electronically.
IIRC, the F100 crash occurred when the T/R deployed, the crew didn't realize why the throttle retarded and were able to force it back forward (basically they managed to override the safety device). In the aftermath we were tasked to determine if was physically possible for the crew to override the strut drum box on the Boeing aircraft that used that feedback (we determined they couldn't).

EMIT, again going by memory here, but I recall for Lauda is that there was a recurring T/R feedback fault to the FADEC, unrelated to actual cause of the deployment (although the troubleshooting of that fault might have contributed - one of the things we discovered during the investigation was the maintenance manual was junk). On the event flight, what we believed happened was the 'reverser stowed' prox sensor was slightly miss-rigged and would occasionally indicated the T/R wasn't stowed - this caused the auto-restow system to open the hydraulic isolation valve which would pull the T/R up tight, the prox sensor would indicated stowed, auto-restow would close the isolation valve. Normal vibration would then allow the T/R to move slightly, the prox sensor would indicate not-stowed and the cycle would repeat. On one of those cycles, when auto-restow opened the isolation valve - for reasons that were never firmly established - the directional control valve changed state to deploy... Whoops:(
The FDR was destroyed in Lauda and there was no usable data - so most of what we know is from the Voice Recorder and the non-volatile fault memory of the event engine FADEC. The FADEC was recording a T/R position feedback fault every time the T/R deployed - so there was a list of 'normal' deploy conditions - 500 ft., Mach .23, 800 ft. Mach .25, etc., then 24,000 ft. Mach .78.
The first time I looked at that fault dump was one of the darkest days of my life...

galaxy flyer

5th Sep 2017, 01:05

sheppey

5th Sep 2017, 02:38

In your aircraft, are there definite indicators that the reverser has deployed,
I can only go by what happens when in- flight thrust reversal is actuated via the instructor panel under the main title of engine malfunctions. And that is the noise caused by the thrust reversal is quite unmistakeable just as you would expect when high reverse is used normally on the landing roll. In the 737 Classic simulator there is very little roll and yaw when the malfunction is initiated but that could be a fidelity issue. In fact it is easily controllable by the average pilot (if there is such an animal)

Basically it says to reduce thrust (Idle), Reduce Airspeed, Shut the engine down, control the aircraft with full rudder trim and sufficient bank to maintain heading.

I don't understand the "full rudder trim" point. What aircraft type are you referring to? Depending on the type of rudder trim but certainly in the 737 Classic it takes 29 seconds to operate the electrical rudder trim from neutral to against its stop in one direction and alone would be useless in terms of controlling yaw. Presumably considerable rudder would be required similar to engine failure in combination of aileron and flight spoilers if needed. Rudder trim would only be needed for subsequent prolonged cruise to reduce leg forces

lomapaseo

5th Sep 2017, 03:03

tdtracer

..... Normal vibration would then allow the T/R to move slightly, the prox sensor would indicate not-stowed and the cycle would repeat. On one of those cycles, when auto-restow opened the isolation valve - for reasons that were never firmly established - the directional control valve changed state to deploy... Whoops

Yes and the critical valve initially could not be found even among the parts being flogged off at the local flea markets. Later I heard that it was found after having been rebuilt in some native's hut.

Your comment about vibration reminded me of the reverser deployment that turned the aircraft over 360 degrees and was only discovered in the data days later with three of us reviewing the DFDR I still remember the remark by one of us as "holy-****" when the roll passed through 180 degrees Again flickering lights. When we tried to duplicate it it wouldn't deploy on the ground during runups (fail safes worked) until somebody gave a rap to the side of the engine with his hand and it deployed (specific vibration floated a fail-safe spring)

lessons learned which I will never forget !!! but now the regs call for three levels of safety .... but it's memories of what we all screwed up that I shall never forget

Vessbot

5th Sep 2017, 03:37

tdracer,

Was it that uncontrollable based on the aero analysis? Why I ask is the C-5 had a TR deploy in cruise and while certainly attention getting, it was controllable and didn't cause structural damage. Yes, at low altitude, not survivable.

All else aside, I would expect a quarter of your thrust reversers deploying to have a less severe effect on controllability than half of them.

tdracer

5th Sep 2017, 03:49

tdracer,

Was it that uncontrollable based on the aero analysis? Why I ask is the C-5 had a TR deploy in cruise and while certainly attention getting, it was controllable and didn't cause structural damage. Yes, at low altitude, not survivable.

Interesting that you brought that part up... There had been some partial in-flight deployments on 747s over the years, and while I suspect it was certainly exciting they landed safely. But aerodynamically the engine installation on a big twin is quite different from a quad.

Initially, the aero analysis said the 767 should be controllable - and in fact they did flight test it during the original aircraft cert - at a stable 200 knots, 10k, engine at idle, they initiated a reverser deployment and were able to control the aircraft.
So we went to the wind tunnel - this was during the development of the 777 and they had a half model of a 777 with blown reverser. The 767 aerodynamically looks very much like a 777, just smaller, so we changed the 777 model from the normal landing configuration to a 'clean wing' cruise with a deployed reverser.
Before we started the test, there was an aero S&C guy that kept proclaiming it was controllable and the flight crew had blown it - going so far as saying he go on a flight test where they duplicated the in-flight deployment. But as the testing progressed he started getting really quiet. By the time the testing wrapped up a week later he wouldn't talk about it...
When they updated the simulation with the updated aero characteristics from the wind tunnel testing, it quickly became apparent the flight crew never had a chance. When they tested the actual deployment scenario, a few pilots could save it when they knew it was coming and it happened during daylight with outside visual queues. When did it at night in the clouds (the actual Lauda conditions), no one could save it even when they knew it was coming :uhoh:
When we finished up with the instrumented wind tunnel testing, they did a flow visualization test - and the result was dramatic. Being a propulsion guy, I didn't really understand all those aero S&C coefficients and such, but I could readily understand the visualization of separated flow. Nearly the entire upper wing was separated, along with a good share of the tail surfaces.

Yes and the critical valve initially could not be found even among the parts being flogged off at the local flea markets. Later I heard that it was found after having been rebuilt in some native's hut.
IIRC, they finally got the Directional Control Valve (DCV) after offering a rather large reward, but it quickly became apparent the DCV had been disassembled then carelessly re-assembled to collect the award, destroying any possible evidence of what caused it to change state.
There are several theories as to what did cause the DCV to change state - one TV show I saw postulated it was a short circuit but most of us involved in the investigation think that unlikely (during testing the circuit breaker always popped before the valve moved). My personal theory is was a hydraulic hammer effect from the repeated cycling of the isolation valve.

galaxy flyer

5th Sep 2017, 04:04

That's very interesting, tdracer. IIRC, they were a former EAL crew on contract to Lauda. Knowing a bit about the efflux problem on the C-5 (engines very inboard compared to the 747) and the mid-ish span location on a twin, I understand what the testing showed.

Goldenrivett

5th Sep 2017, 07:14

Hi sheppey,
I don't understand the "full rudder trim" point. What aircraft type are you referring to?
A320. From FCOM below:
"If Buffet:
The warning alone, without buffet or vibration, may be a false warning.
MAX SPEED... 240 KT
ENG MASTER... (affected engine) OFF
If reverser is actually deployed:
RUD TRIM... FULL R (L)
CONTROL HDG WITH ROLL"

In Normal Law, it may be due to fact there is no feed back through side stick of the amount of aileron required and possibility of running out of aileron authority unless lots of rudder is applied.

EMIT

5th Sep 2017, 07:31

Very interesting details, tdracer!

As far as training is concerned, when the accident happened, apparently the simulators still had a failure mode of REV deployed inflight. After the modification of the real 767 aircraft system, so that REV deployment inflight was really moved into the 10 to the minus umpteen regime, the failure mode was removed from the simulators. By the time I had a sim session and wanted to "try" whether an inflight deployment would be recoverable, it was no longer possible to have the failure simulated.
Reading the replies from tdracer, I assume the simulator fidelity of the REV deployed inflight scenario would not have been realistic anyway.

Jwscud

5th Sep 2017, 08:27

We used to train this in the sim on the Lear 45. The thrust lever automatically snapped to idle if the reverser deployed and we were trained immediately to shut the engine down.

The aircraft was marginally controllable and between stick shaker and V2 until you shut it down. You could just about attain a positive rate. One wily TRE also trained knowing the location of and instinctively pulling the deploy CB (4th back on the bottom row since you ask :E) as a backup.

It was a good sim exercise, but it was seriously demanding even if you were in "sim" mode, and over beers we wondered whether it would be survivable if it happened for real.

paradoxbox

5th Sep 2017, 16:00

So do people think it's worth the risk of setting power to idle without knowing for certain if a reverser is the cause?

Some people have mentioned that the vibrations and noise would indicate a reverser problem, but in the aircraft I have flown, spoilers also cause a lot of vibration and noise and yaw/roll action. Without being able to see the back of the aircraft I think it would be hard to differentiate between an uncommanded reverser and an uncommanded assymetric spoiler deployment very quickly - unless you had some kind of experience, sim or otherwise, that let you judge otherwise.

I agree with the guy who mentions combat / aerobatic training as being extremely useful in this kind of situation. I think that a typical civilian crew that has "positive g, altitude" on their minds 24/7 may be in trouble. Unloading the aircraft (0g) would probably buy you a lot of time.

I wonder how unloading the aircraft (0g / 0 or low AoA) would affect things in a thrust reverser deployed situation. I do not trust sims to get this right but I also do not want to try it in a real aircraft, thankyouverymuch. Any test pilots with parachutes want to give this a shot?

underfire

5th Sep 2017, 16:43

lomapaseo

5th Sep 2017, 16:45

So do people think it's worth the risk of setting power to idle without knowing for certain if a reverser is the cause?

Risk ??? of what?

In an actual event, the crew reported flickering unlock lights during initial climb. They discussed the possibility of false warnings (loose connections, wiring etc.) but decided it would be prudent to follow the FCOM and pull the affected engine back to idle. The unlock light ceased so they once again advanced the throttle to maintain climb profile. The reverser then deployed and rolled the aircraft over to 180 deg in a matter of seconds. The airforced trained PF then completed the roll through 360 deg and requested clearance to return to the field.

On ground examination found nothing wrong.

underfire

5th Sep 2017, 17:00

In an actual event, the crew reported flickering unlock lights during initial climb. They discussed the possibility of false warnings (loose connections, wiring etc.) but decided it would be prudent to follow the FCOM and pull the affected engine back to idle. The unlock light ceased so they once again advanced the throttle to maintain climb profile. The reverser then deployed and rolled the aircraft over to 180 deg in a matter of seconds. The airforced trained PF then completed the roll through 360 deg and requested clearance to return to the field.

WOW, what ac was this?

Vessbot

5th Sep 2017, 17:08

Interesting. I seem to remember a few aircraft that would use deliberately use the thrust reverse in flight, the DC8? The Concorde to get from SS speeds.
It is commonly used on a few military aircraft, I remember the ride on a C17 diving in...without warning, it does feel as if you have been shot down. you could not do that with paying customers, at least ones you like.

In those cases the TR deployment is symmetrical, starts at idle, and power goes up to some predetermined limit that's flight tested and certified to not pose any issues wrt. efflux blanking out controls, etc. Not sudden, on one side, and at cruise or higher power.

TURIN

5th Sep 2017, 17:56

First of all, thank you to all the contributors to this thread, I've not been this gripped since the Concorde thread started.

lessons learned which I will never forget !!! but now the regs call for three levels of safety .... but it's memories of what we all screwed up that I shall never forget

There for the grace of...etc.

I'm still amazed at how languid some pilots/maintenance organisations are about T/R faults. After the Lauda incident, which I remember well, I had occasion to lock out faulty T/Rs on Boeings and Airbus' and the attitude of some was quite disturbing.

blind pew

5th Sep 2017, 18:23

On the trident we used in without any problems although it wasn't very pleasant down the back.
Think the limit was around 11,000 but we were allowed to use emergency reverse in the flare.
The dc8 was reverse on inners.
In the mid 70s Concorde had an engine go into reverse in cruise. It returned to lhr, got another crew and after trouble shooting was sent on its way only to have the same fault after rotation.
It crossed over my cottage, near to Newbury, so low that I rushed outside thinking there was a crash; about 40 track miles; it only got above three grand crossing the Bristol Channel. Iirc the engineeer got some sort of award for shutting the engine down velly quickly.

Chu Chu

5th Sep 2017, 22:38

It sure sounds like a good thing the DCV was carelessly reassembled -- if it had be done carefully but with one "small" oversight, it might have really ruined someone's day . . .

josephfeatherweight

6th Sep 2017, 03:03

This is a brilliant thread - some very knowledgeable contributors, for which I am grateful.

RAT 5

6th Sep 2017, 12:18

The airforced trained PF then completed the roll through 360 deg and requested clearance to return to the field.
On ground examination found nothing wrong.

Ah, the famous "ground tested found satis" tech log reply. I wonder what the FDR showed. That would have been interesting. Indeed, following the a/c all the way through the manoeuvre, rather than reversing it, might sometimes be the best action. Not recommended with looping high nose attitude though.

I can't remember accurately the follow up to Lauda. They went in the sim and experimented and I think they all failed. I don't recall if they tried just closing both TL's immediately to gain control and descending. In fact, with the design of the day, would they have ended up with one at forward idea & one at reverse idle? I suspect the roll rate would have been eye watering and uncontrollable with rudder & aileron, but with both at idle, what then? There were a/c with too small rudders, (VMCA) when with an engine failure, and the other at full power, is was necessary to reduce thrust to gain control. Not the first instinct unless trained.

paradoxbox

6th Sep 2017, 17:34

There were a/c with too small rudders, (VMCA) when with an engine failure, and the other at full power, is was necessary to reduce thrust to gain control. Not the first instinct unless trained.

This is golden info, thank you very much.

Someone asked me "Risk of what?" when I queried as to whether reducing thrust is safe to do without a definite indicator of thrust reverser deployment. The risk I imagine is making your energy state worse on takeoff or climb by reducing throttle on one or both engines before you know exactly what the problem is.

In a few incidents I can think of, the crew instinctively shut down either the wrong engine, or shut down an engine still providing forward thrust, just a little after takeoff.

There was a Taiwanese Dash 8 Q400 that crashed because of this. The aircraft had been experiencing engine trouble on previous flights and assumed that it was the same engine causing the problem this time. The pilot shut it down without confirming that it was indeed that engine experiencing the problem. In actuality it was the other engine which was not performing correctly this time and he shut down the only engine still operating normally.

Another incident I can think of is the Concorde crash, where the flight engineer shut down an engine (or both? Can't remember) because of the fire warnings going off. That engine was still providing thrust and if he had not done that, they may have had a chance of getting more speed and altitude. Of course they were in trouble due to the severity of the fire burning through the wings, but if the engine had not been shut down they may have had more time to deal with the problems, possibly saving the aircraft.

Those types of incidents make me very wary of cutting throttle in takeoff or climb before knowing what is causing the problem for certain.

Is it safe to assume that if the reversers do deploy for whatever reason, we can expect reliable indicators and should just reduce thrust? Again, would unloading the wings save you from uncontrollable roll at lower airspeeds like 190-230 knots? Perhaps lower wing loading would give you enough rudder/roll authority to at least look for other possible causes.

lomapaseo

6th Sep 2017, 18:47

Someone asked me "Risk of what?" when I queried as to whether reducing thrust is safe to do without a definite indicator of thrust reverser deployment. The risk I imagine is making your energy state worse on takeoff or climb by reducing throttle on one or both engines before you know exactly what the problem is.

The thrust reverser control problem has been at it's most difficult to control when some control surfaces were inhibited in later climb stages. If this was the case where you were asking about the possibility of inappropriate engine reduction then I would judge it not to be of significant risk since your aircraft normally has plenty of ground clearance and control speed to maintain safe flight.

I am however sensitive to quick reactions by the crew in pulling back wrong engines and as such far prefer automatic (via FADEC etc.) reductions and not hard shutdowns.

Of course I expect pilot training now covers all these concerns quite well.

underfire

7th Sep 2017, 00:47

In those cases the TR deployment is symmetrical, starts at idle, and power goes up to some predetermined limit that's flight tested and certified to not pose any issues wrt. efflux blanking out controls, etc. Not sudden, on one side, and at cruise or higher power.

Understood, but as the aircraft had procedures to deploy reverse thrusters inflight, it seems reasonable that procedures were in place if only one of the pair deployed? This may be a good learning experience on that adventure? (ie on the Concorde, when using rev thrust to slow from supersonic, and only one deployed, what was SOP, that seems a bit of an extreme case)

galaxy flyer

7th Sep 2017, 03:46

Again, would unloading the wings save you from uncontrollable roll at lower airspeeds like 190-230 knots?

I would think unloading wouldn't increase control because the problem is the reverse efflux is blanking out the flow over the wing and even zero G won't change that.

paradoxbox

7th Sep 2017, 05:34

Does this apply to aircraft with tail mounted engines as well, to the same extent as with wing mounted engines?

tdracer

7th Sep 2017, 15:57

I've never spent significant time working an aircraft with tail mounted engines (I was responsible for the 727 for a while, but it was decades out of production at the time and there wasn't much going on). However my engineering judgement says that with tail mounted engines, the problem is going to be dramatically reduced tail effectiveness due to the reverser efflux blanking the tail control surfaces. So it would be a fundamentally different problem than the one faced by Lauda where the aircraft almost instantly rolled into a dive due to the dramatic loss of lift on one side.
My understanding of the F100 crash (where the reverser deployed shortly after takeoff) was they would have been OK if they'd just left the engine alone after the feedback pulled the throttle to idle. But somehow they managed to brute force the throttle back up to high power and that's when they lost control.

BTW, the FARs (25.939) still don't mandate the 3rd level of protection, still spelling out that an uncommanded T/R deployment must be controllable. To certify, Boeing has had to petition for an "Equivalent Level of Safety" (ELOS) - basically showing that it's as safe as if the aircraft was controllable because it's shown it'll never happen. Providing an ELOS is a royal pain in the rear compared to showing direct compliance. EASA is better - they updated the corresponding CS to say that you either have to show it's controllable - OR - demonstrate that an uncommanded deployment is "extremely improbable" (which is defined as 10E-9 hr. or less).

galaxy flyer

7th Sep 2017, 16:09

No, it doesn't, the efflux is too far aft, depending on specific design. It's controllable in sim profiles on the Challenger and Global types bug more yaw that simple engine failure. I can't say as to the fidelity of the aero data, however.

The tail-mounted designs usually have a T-tail, so above the efflux. The Falcons with cruciform tail design only use a reverser on the center engine. Whether that is related to uncommanded deployment I cannot say.

In the C-5, it was inboards only for in-flight use. Effective, but I wouldn't use them below 10,000' so as not to get caught with a hung up reverser.

t7a

7th Sep 2017, 17:59

tdracer is absolutely correct when he says that the Tarom F100 crew would have been OK if they had not forced the thrust lever open (3 times) and then shut the engine down. After this accident our sim check rides included uncommanded reverser operations (normally just after Vr). Although it was an eye-watering experience the aircraft was controllable provided the engine was immediately shut down.

RAT 5

7th Sep 2017, 18:49

I think the idea of handling, successfully, uncommanded open TR's in flight by reducing thrust is going to depend on height. If climbing through FL 240 converting your a/c into a glider and sorting out any roll problems is a possibility if PF has some aerobatic experience. If there is enough height there is no need for high g manoeuvring, but controlled inputs, and as someone said, let the a/c roll all the way round and catch it on the way out. Doing this at idle thrust removes a few of the very severe problems. Meanwhile the airspeed is off the clock, but hey, it's still flying. Steady the wings, gently pull out of the dive ignoring the EGPWS, trade see for height and calmly decide which is the engine to use to stabilise the a/c under control.

Then the dream is over and I wake up to loud applause and take a bow, with my wife saying WTF.

bafanguy

8th Sep 2017, 16:40

Speaking of T/Rs deployed in flight, this promo has an interesting bit at 5:28:

https://www.youtube.com/watch?v=fMyHz33X5Ko

EMIT

12th Sep 2017, 02:58

Quote
Again, would unloading the wings save you from uncontrollable roll at lower airspeeds like 190-230 knots?
Unquote

Galaxy Flyer
Quote
I would think unloading wouldn't increase control because the problem is the reverse efflux is blanking out the flow over the wing and even zero G won't change that.
Unquote

If the efflux over the wing with uncommanded thrust reverse causes massive loss of lift over that wing, then the “normal” amount of lift over the non-effected wing will cause a roll, against which the normal roll controls are not powerful enough.
Unloading to zero g will not magically make the normal roll controls more powerful, but it will reduce the lift on the “normal” wing to zero as well. In that condition, the normal roll controls will be able to supply normal roll control.

Problem with zero g is, (one), passengers and cabin crew will float through cabin, if not strapped in and ,( two), zero g will cause a ballistic path towards the earth, so it cannot be maintained forever. However, a temporary use of zero g may allow roll control during the time that is needed to reduce thrust to idle. Once thrust on the bad engine is at idle (or the bad engine is shut down completely), normal flight with enough normal control effectiveness should be possible again.

For the Lauda accident: the fact that the event happened at night, in cloud, should not have made any difference: the big attitude display of the 767 should have been sufficient for recovery from any unusual attitude. (this remark is in no way meant to detract from the magnificent information given by tdracer, nor meant to criticize the Lauda crew).

galaxy flyer

12th Sep 2017, 03:05

EMIT,

Quite correct, I hadn't thought thru the implications of reducfing lift on the unaffected side. I'd guess night, zero G maneuvers and instructing the PM to shutdown the engine would earn one a early retirement.

tdracer

12th Sep 2017, 05:24

EMIT, I believe the key was they had to respond to the T/R deployment immediately - and lacking the outside visual queues and making sense of the attitude display took a second or two that they didn't have.
The FADEC fault dump showed faults consistent with the aircraft breaking up ~10 seconds after the T/R deployed (and with going supersonic before that). It happened fast.

Disclaimer - I wasn't involved in the simulator testing itself, I'm simple relaying what I was told by those who were...

pattern_is_full

12th Sep 2017, 14:15

@tdracer - yet another thanks for the insight into the loss-of-lift dynamics in the 767.

Did Boeing apply any "lessons learned" from the 767 lift-blanking to the 737 classics and NGs, as the larger engines required a location change from under-wing to "forward and upward?"

I realize the Classic design was already in production (1984) before the Lauda Air accident (1991). I just wondered if there were 737 retrofits of extra interlocks added post-Lauda, or if the 737NG already had those, or if the thrust dynamics are just different even with the more "767-like" engine position in newer 737s.

paradoxbox

12th Sep 2017, 16:12

so, TR deployment @ near max cruise seems to be an extremely dangerous situation with seconds separating recovery from breaking the sound barrier and your plane.

airbus and boeing seem to have taken care of it with electronic wizardry - not that i trust it farther than i can throw it. 10E9 probability with my luck it'll happen to me a dozen times on the same flight. At cruise if you experience sudden extreme roll is it a safe bet to slam the throttles to idle? Levitating PAX be damned if it means you won't die.

I would like to know if anyone has done any test flights to determine the roll rate at various airspeeds during thrust reverser deployment at different power settings.

Back when dinosaurs lived, I did aerobatics and was a huge fan of 0 or negative G (or low AoA) to get myself out of all kinds of bad situations. Of course jets are very different from props but I cannot help but think that there must be a way to buy time in this situation. With the aerobatic planes I flew, unloading the aircraft generally resulted in superhuman maneuverability, no doubt the propwash playing a part but I would imagine that even in larger jets, you might be able to get some roll rate back either through rudder or aileron if you unload. Depending on the g load necessary to restore roll the amount of time you could gain could be significant especially if you have a monitoring pilot to help. Keep in mind I am thinking primarily about lower airspeeds here, as it seems that at cruise speed your only choice is to cut the throttle immediately or end up inverted and passing mach 1.

I'm not entirely convinced that a TR indicator is going to show up on the engine screen and neither am I convinced that I am going to be able to hear or identify that it's a TR if I am in a large jet with the cockpit a mile away from the engines.

lomapaseo

12th Sep 2017, 18:25

I would like to know if anyone has done any test flights to determine the roll rate at various airspeeds during thrust reverser deployment at different power settings.

It's hard to get pilots to fly a test like this.

They much prefer some lesser tests and lots of extrapolation.

at any rate airspeed is your friend as it destroys the reverser efflux and sensitizes the wing control surfaces to work as planned.

Of course nothing is 100% so the FAA now requires a few more hoops to jump through before saying that it's good enough.

tdracer

12th Sep 2017, 22:15

I realize the Classic design was already in production (1984) before the Lauda Air accident (1991). I just wondered if there were 737 retrofits of extra interlocks added post-Lauda, or if the 737NG already had those, or if the thrust dynamics are just different even with the more "767-like" engine position in newer 737s. My understanding was that in the aftermath of Lauda, ALL the then in production Boeing aircraft added the third lock (aka "Sync Lock" since they locked the synchronizing shaft that ties the actuators together to make sure they move at the same rate and don't jam the sleeve) - either in production or by AD mandated retrofit. That included the "classic" 737-3/4/500 although I don't think it was extended to the "Jurassic" 737-1/200.
A big part of what Boeing learned in the aftermath of Lauda was how dramatically the move to ever high bypass engines - hung out in front of the wing - changed the dynamics of a T/R deployment in flight.

airbus and boeing seem to have taken care of it with electronic wizardry
Paradox, what electronic wizardry are you talking about? At least on Boeing every new aircraft has a third physical lock - enabled only on the ground - that will prevent even a powered T/R deployment. On the newer Boeings, in part because the sync lock has some undesirable failure characteristics (it's difficult to tell if it's failed without actually testing it) they went to a track lock - a physical block in the T/R slider track that will prevent reverser deployment, the health of which is easily monitored with prox sensors. If you're really unlucky enough to have an uncommanded T/R deployment in-flight on a Boeing equipped with the third lock, I'd suggest you never go outside because if you do you're going to hit by lightning....twice....on the same day :rolleyes: