Air Crash Investigations BM 737-400 Crash
 

Hi,

Could any B734 pilot or LAME enlighten me on this one as I've missed this bit on the show. My question is posed as to why is it that when they've throttled back the right engine the vibration on the left engine stopped.

I'm aware that they've disconnected the A/T before throttling back the right engine so that effectively stopped the left throttle from compensating to maintain speed (making the shuddering worse) but it shouldn't have stopped the excessive vibration - but it did !

Thanks everyone

8Ball

Hi this is taken from the documentary..

"When the fan blade fractures on the left engine, it's fragments damage the engine severely causing a reduction in fan speed. The a/t reacts by pumping more fuel to the damaged engine trying to maintain the fan speed. This causes the damaged engine to violently vibrate and throw out sparks, these ignite surface fuel and cause the flames seen by the pax."

"When the pilots d/c the a/t in order to shut down the right hand engine, both engines return to manual throttle. Sensors inside the damaged left engine detect that its spinning slower than normal which triggers a reduction in fuel flow. With the fuel flow now at a level the engine can handle the vibrations and flames disappear."

Hope this helps.

FS.

Huh ??
 

"Sensors inside the damaged left engine detect that its spinning slower than normal which triggers a reduction in fuel flow"

This means that as the a/t is d/c the fan blades spin slower than they would be at the position the throttle is left at due to the debris. Therefore the a/c has sensed the loss of power in the spinning blade through its sensors and in response reduced the fuel flow to that engine. In brief, the a/t was forcing the blades to spin to create power for that engine, whereas when the a/t is d/c although the throttle is still where it was, the a/c has sensed a slower rate of spin and automatically decreased the fuel flow. Basically the a/c reacted as if the pilot commanded a decrease of power in the left engine through a series of automatic sensors, when he had not and shut down the incorrect engine.

Hope this helps.

FS

I missed the program, however another reason the vibration went away could be because they commenced descent to their diversion port, and had idle thrust - or reduced thrust on the damaged engine.

When they powered up on final - back came the vibrations, and it eventually sh!t itself.

Felix,
You have probably accurately quoted the documentary (I missed it), however their explanation is quite illogical.
All the A/T ever does is move the thrust levers. Fuel flow to the engine is controlled by the Electronic Engine Controller (EEC). The EEC receives several inputs, a prime one being thrust lever position; note that A/T on or off has NO effect on EEC commands.
If fuel flow reduced when the A/T was disconnected, this could have been due to a manual movement of the thrust lever. Or, it could have been coincidental timing, with the EEC finally realising at this stage that fuel supply was excessive, or the engine RPM finally winding down enough (due damage) to reduce vibration, or....
My point is that disconnecting the A/T cannot in itself cause a decrease in scheduled fuel flow to an engine. Which brings us back to the question ... why did the vibration stop (about the time the A/T was disconnected)?

Neither LAME nor pilot (not airline a/c, anyway) so please excuse this post, but it may be useful. Just done a HF course in which this accident was discussed in some detail, and a film shown. Don't know if it's the film you refer to, though.
I don't have the level of tech expertise in this field to answer, but what the tech people were saying about it was that the EEC detected a drop in thrust following the damage, and compensated by throwing more fuel in to maintain thrust. With the imbalance and damage, this resulted in flames/bangs/excess vibration. At this point the crew had no idea which engine was at fault. Their prior experience was with a/c in which the vibration guages were known to be unreliable; the (brief) conversion course did not point out that the guages in this a/c were superior and accurate. Throttling back the right engine was pretty much guesswork, confirmed in their minds when the vibration stopped. At that point the EEC detected the thrust lever mismatch and disconnected; fuel flow returned to normal in the left engine, reducing the vibration. Descent was commenced for diversion aerodrome shortly afterward, reducing the strain on that engine until it was powered up again on approach.
Several other factors including r/t workload which effectively interrupted their checklist following/during the shutdown, poor location and "attention-getting" characteristics of the vibration gauges + lack of any other associated warning device, and the design of that engine, which was basically a re-rated version of the 733 powerplant, and had not been properly tested at altitude.(It was the torsion produced in the blades at the higher rpm that occurred only at altitude that lead to fatigue.) I understand there were at least 4 similar blade failures in this engine.
Hope that helps.

If the left engine now ran with less fuel= less power why didn't the Air speed drop or did it and they didn't notice

Don't know. But I'd imagine that it did drop, and since they had just throttled back the right engine they expected it to drop, and they commenced descent not too long after that, anyway.

Multi-engine procedure
 

I don't know if it's applicable to jets but in our multi-engine assymetric drills, we power up the remaining live engine as soon as the other quits and we try to verify and confirm and by that, this should've given them a clear indication which engine was in trouble. Throttling up the problematic left engine (and throttling back the right) would've made more noise and that should've given it away.


Just my 2 cents worth. It's probably easier said than done but I would've done it that way.

Thinking?
 

That requires thinking, which is not permitted. You must follow the checklists only.
This is yet another case of crew shutting down the live engine. It has been happening for more than half a century.
One important thing. If this TV presentation is accurate, they did not take notice of the vibration indicator, which would have told them which one it was, partly because they did not consider them reliable.
How many aeroplanes have run out of fuel after the fuel guages had been showing empty and been ignored because they were considered unreliable?
They do not fit unreliable things to aeroplanes.

Please tell me you're being sarcastic.

• Engine fire detection system on Kingair
• Fuel gauges on almost anything < 5700 kg, particularly BN2, N22, N24, PA28
• Almost any valve on a BAe146
• Door warning system on Kingair
• Coffee brewers on B717

????
 

See what I mean?..................................
We did not have any of those problems in our King Airs.

At least they missed tins favourite boozer :ooh:Ye Olde Flying Horse Inn

Coffee machine?
 

One of our(single pilot) KingAirs actually had the coffee machine listed on the pre takeoff checklist!!

I recall that the vibration indicators on the 737-400 were EFIS type (not steam driven as in the 737-200 series). Now if the vibration level gets excessively high and the needle swings up and around to max stop, the design is such that the needle concerned actually disappears from the instrument dial. Rather similar to the glide slope indicator on EFIS aircraft that is blanked from view until it detects a useable GS within certain localiser angles off.

One of the results of this vanishing vibration needle is that the good engine indicated the normal vibration level (low) while the defective engine vibration needle showed nothing because it had taken itself off the air due severe vibration. This was partly why the crew mis-identified which engine had experienced problems.

I am no B737 expert either, however, there are a few of slightly incorrect assumptions being made here.

markjoy said, "...At that point the EEC detected the thrust lever mismatch and disconnected...". I don't know that any EEC would do this. Each engine has it's own EEC and they work quite independently so would not do anything in the case of a thrust lever mismatch.

Maybe you're talking about the Autothrottle? As Avid Aviator says, "All the A/T ever does is move the thrust levers.". Quite correct. The EEC is the computer that sits between the thrust lever and engine and tries to get the engine to do what the thrust lever is commanding, regardless of whether the autothrottle is commanding the thrust or the pilot.

The autothrottle (when engaged in the cruise situation) would have been moving the thrust levers to command a particular speed. When actual speed goes down below commanded speed, the autothrottle pushes thrust levers forward to regain the commanded speed. In cruise this is not particularly aggressive as this results in an uneconomical fuel burn i.e. if actual speed is M.01 below commanded speed, the A/T does not "pour on the coals" but gently increases thrust. It would be more aggressive if the speed shortfall was larger.

The fuel flow that results from a particular physical thrust lever setting will be determined not just by the actual thrust lever position but also by aircraft weight, centre of gravity, altitude, temperature and airspeed. EEC's can, and do, fail of course (despite what bushy says!). In fact aircraft can be dispatched with EECs inoperative (just be careful firewalling those thrust levers!). I am not sure if there were any problems with the EEC as a result of the engine spitting a blade in this case, however anecdotal evidence would suggest there was not.

Not trying to be anal, just be careful with your assumptions.

To answer the original post about why the vibration reduced when they reduced thrust, here is an analogy. Turn your pushbike upside down in the garage and spin the wheel really fast. All good? Now stop the wheel and give the wheel in question a few nice belts with a big mallet. Spin the wheel really fast and the bike will shudder like buggery and probably fall over, however spin the wheel slowly and it won't actually vibrate that much. Apply the same principle to a series of big metal discs rotating at tens of thousands of RPM in an engine being forced to turn by combusting fuel and air. When an engine is brought back to idle it is, essentially though not quite, windmilling so the vibration would have decreased by a substantial amount.

The whole reason the autothrottle would have been disconnected by the crew is to reduce the vibration and although this action reduced the vibration it made subsequent diagnosis of the problem more difficult as the problem appeared to go away. Believe me, it comes as quite a shock when this sort of thing happens - lots of noise, vibration, calls from the cabin crew etc etc.

It would appear there were a number of other factors contributing such as: limited and therefore possibly inadequate conversion training for pilots, both pilots limited experience on type and poor design of engine instrumentation. I recall the captain was interrupted by a radio call as he was about to review their decision to shut down the "wrong" engine and subsequently did not complete the decision making process.

This particular variant of the CFM engine was not tested in all flight envelopes (combinations of airspeed and altitude) prior to entry to service and subsequent to this accident more stringent certification requirements (as far as testing goes anyway) were introduced for engines.

With the benefit of 20/20 hindsight (!), lessons learned are.... Sit on your hands unless you are on fire or about to hit hill, don't let ATC rush you, use ALL the information available to you to make a decision and when you've made a decision, go back and review that facts and figures to make sure you did in fact make the right decision.

Safe flying people.