closing the thrust levers too quickly at high altitude?
 

Hello,
some time ago I had a captain complaining that I closed the thrust levers too quickly at relatively high altitude [3 seconds maybe - 25000ft - 737 NG - cfm 56-7].

Never found any limitation or best practice about this topic in the manuals...And I think that the eec would regulate the fuel flow to allow the engine to stay within acceptable parameters!

Excluding pax comfort and just from a technical point of view: do you have anything to share guys?

ps: think the last time you did the emergency descent in the sim.. were you gentle with the thrust levers?

With a FADEC you need not worry but with FCU engines slam decelerations can lead to flameouts if the deceleration stop is out of calibration.

Some years ago CFM came and gave a presentation to the trainers and tech pilots where I work, the advice they gave was that under normal circumstances the thrust levers should be moved at a rate no quicker than the auto throttle system moves them.

From a purely fuel-conservation standard, a slow ~30sec. decel is best; also kinder from a pax comfort angle.

I have had DEEC engines surge with over-enthusiastic thrust lever closures at altitude.

The surge resulted in a BANG as the engine slowed.

Do you really want to be telling the FADEC "This is an emergency! I need minimum power as soon as you can do it!" - or do you want to ease the thrust lever back, effectively telling the FADEC "Hey, relax dude - this is a normal deceleration for descent." ;)

Do you want to be training yourself to only be able to fly FADEC engines? .. or would you like to train yourself proper piloting techniques, so that you aren't reliant on computers correcting your poor flying? :8

(For those not as old as I am, DEEC = Digital Electronic Engine Control, a digital computer linked to a basic Mechanical Fuel Control - a "poor man's FADEC")

From an airmanship point of view I totally agree with your point!
But I was trying to understand if there was also any technical limitation involved.
From the responses it seems to be allowed with no restrictions, even if it's not advisable in normal ops.. thanks guys!

I always tried to look after the engine as best I could, and that meant the power reduction to idle at TOPD was done quite slowly to let the temperatures change nice & smoothly.
It won't hurt the engine by chopping the throttle to idle but it can't help the longevity of them. (if that makes sense)

Same deal for when I used to fly little turboprops around - when taxiing in after landing I'd try to keep the power changes to a minimum so as to keep the temps as low as possible for as long as possible before shutdown.

I can see your point of view clearly. But what you have seen in the replies is nothing more than personal opinions. Facts are another matter. In a car if you wish to slow down you simply lift your foot from the accelerator pedal. There is no question if this should be done slowly and deliberately in order to let the engine revs run down for perceived `good` engine handling. Of course not. You simply remove foot pressure. Same with closing the thrust lever for descent or at the flare for landing. I remember my old Boeing instructor from Seattle saying at the flare he wanted a sharp cut and hear the sound of metal hitting metal as the thrust levers hit the stops. None of this slow cut business which only extended float and made no difference to engine handling.

Rather sure he was trying to make a point across,that reducing the thrust too slowly will increase flare and also higher risk of bounce landing.
However since the thrust lever angle being already low entering the flare area,a standard continuous reduction in thrust or an abrupt will achieve the same.
No need of slamming banging i think.

Big pet peeve: slamming the levers to flight idle.

Absolutely no need for that at any point in the flight.

Any engine manufacturer that makes an engine today had better be sure it is impervious to throttle jams.

I don't mind an overt recommendation from an installer either way in a quest to fly the aircraft most effieciently, but if the engine breaks the engine guy is going to have to fix it.

It's not in the interest of safety to have pilots having these kind of questions in-flight

Tee Emm - Call me pedantic, but I do believe there is a difference in operation between an Otto cycle engine and a Brayton cycle engine.

It's not about worrying about the mechanical parts so much as it is about controlling the high pressures within the engine, and preventing reverse flow under rapid deceleration, and possible flameout.

... and its implications regarding pax comfort reflected in ECS,

I was told in early training it was to avoid shock cooling of the engine typically at TOD at high altitude. However, I thought this was piston engine stuff (cyliner damage, etc.)

No problems for the motors as the EECs take care of scheduling fuel inputs to the HMFC. However the PSRVs may not react to fast thrust changes thus causing bumps and discomfort.

Early on in my career I was told the very same thing ref thrust lever closing at altitude. The Instructor pilot explained that with the decrease in RPM also comes a change in exit guide vane angle so as to keep undisturbed flow thru the core. These were PW4056's and I think the FEGV's were scheduled with fuel muscle (happy to stand corrected) and were more suseptable to surging at alt than the RB211's that we also operated.
Perhaps this is where your answer lies.
Rgds

What about a ULR flight where the engines have been stable at 750 odd degrees for 16 hours and thrust levers are abruptly closed...? That can't be good for any metal in my view.

me thinks that too much is being made of metal temperature changes.

These engines are tested within this environment (snap decels) and mostly vetted. The metal temp doesn't change as nearly as fast as the gas temp and the engine parts most susceptible to these gas temp changes are designed to be mostly impervious by virtue of not continuous hoops

On the 737NG just because you command the engines to do something, does not mean they will respond instantly. You will see the "command sector" on the DU to reflect your input and the EEC's will then control the thrust retardation to avoid shock cooling or other unwanted consequences.

As the other posters allude, it is not a good habit to get into. I can't think of ANY situation where you would need to slam home the throttle levers, rather than slowly bringing them back. One day you will fly a type which may not have such good protection and it's good to learn sound operating practice now rather than later!

P.S. Apologies for thread bump didn't realise this was two weeks old!