Thrust reduction during departure

[ex902]

Hi,

We are working on a paper focusing departure and climb phase. According to data for typical narrow-bodied commercial aircraft we have, it seems that the engine thrust is reduced at about 1500 ft. For instance, the average N1 is about 88.6±2.6% between 35 and 1500 ft, whereas it is about 87.7±1.8% between 1500 and 3000 ft. There are noticeable decreases at other engine/aircraft parameters as well (i.e., FF, EGT, climb gradient etc).

If you agree with my explanation so far, I have two questions:

How does the presence of obstacles at the departure track affect the thrust reduction height? Could you give me an example please?
Do all aircraft execute thrust reduction at a specific height? Or it depends on performance of aircraft, weight, weather conditions or departure procedure?

Many thanks in advance.

[tttoon]

Hey,

Thrust reduction (and acceleration) heights are airport specific, not operator or type specific. Each airport can prescribe a certain noise abatement procedure. You can read up on ICAO NADP A and B for the specifics.

Noise abatement procedures are not dependent on obstacles, performance, weather, etc. With all engines operating aircraft have no issues obtaining obstacle clearance in 99% of the airports. In case of an engine failure, special procedures are flown to assure obstacle clearance.

[tom775257]

If you require to increase all engine thrust reduction altitude due to obstacle clearance, what are you meant to do if you lose an engine? Now there may well be airfields where you do have to do this to achieve all engine terrain clearance and have a specific escape procedure if you do lose an engine, but I haven't come across one yet.

I would perhaps expand on the comment above with regards to stating that thrust reduction altitude isn't operator specific. We have standard operating procedure written down (UK airline / A320) that we shall both reduce thrust and accelerate at 1000' AAL unless stated otherwise. Yes it is true that certain airports ask you to modify these for noise abatement in which case we do, but for most destinations I fly to, we don't modify this. More commonly we increase the acceleration altitude though due to obstacle clearance (but keep thrust reduction at 1000' AAL).

The only case I have come across where thrust reduction altitude is increased is specifically for noise abatement (more noise closer to the airport, less further away as aircraft has achieved a greater climb gradient).

In summary I would suggest for obstacle clearance you should be thinking more about acceleration altitude than thrust reduction altitude. Acceleration altitude is where we lower the nose, accelerate to our climb speed whilst bringing in slats/flaps.

[Escape Path]

Departure gradients are based on OEI, hence why you will meet the gradient with AEO in most cases. I don't recall seeing a procedure that has a climb gradient for AEO. If you can't meet the gradient on OEI, you will need an EOSID.

Your thr red/Accel height of 1000ft meets the minimum of 400ft, so yes, it's operator defined but meets regulatory standards. For airports with NADPs you will need to modify that if your operator standard doesn't meet the NADP (our case, Colombian airport with Accel height of 3000ft agl)

[GlenQuagmire]

Pretty sure its the other way around... The published gradients are all engines operating requirements. If you can make those gradients with one engine inoperative then you dont need an escape route. otherwise, you do..

[172_driver]

How does the presence of obstacles at the departure track affect the thrust reduction height? Could you give me an example please?
Do all aircraft execute thrust reduction at a specific height? Or it depends on performance of aircraft, weight, weather conditions or departure procedure?

Q1: it doesn't.
Q2: thrust reduction heightis an airline SOP

the two answers above refer to a routine departure with all engines operating. If you've got an engine failure prior to thrust reduction height you continue with take-off thrust until the flaps are retracted starting at "engine out acceleration altitude" (or the AFM limitation of 10 minutes on our 737.. if you remember)

As a side note, what you call thrust reduction height often becomes a "thrust increase height" as we derate so much that our take off thrust is lower than climb thrust :)

[Escape Path]

Pretty sure its the other way around... The published gradients are all engines operating requirements. If you can make those gradients with one engine inoperative then you dont need an escape route. otherwise, you do..

I think we are saying the same thing, English is not my first language, so pardon my confusion.

@172_driver

I thought one of the flex requirements was that it couldn't be lower than climb thrust. Unless you are strictly speaking of derate only, in which case nevermind me.

[LW20]

In our company the standard departure procedure is thrust reduction and acceleration in 1000 ft AGL. Some airports may require higher values, sometimes only on specific runways.
E.g.: RKSI: Take Off on Runways 15 L/R, 16: 1000ft/1000ft
Take Off on Runways 33L/R, 34: 1500ft/3000ft

[Capn Bloggs]

Take Off on Runways 33L/R, 34: 1500ft/3000ft
Curious, why the higher altitude?

[8314]

Noise (1500/3000 = NADP1)

[RAT 5]

There is a difference in philosophy between thrust reduction height & acceleration height. B738 thrust reduction is automatic at 1500'. No where have I found an operator's differing SOP. I was told decades ago that RTOW tables calculated obstacles up to 1500', hence the thrust reduction was designed for that. All engine acceleration depends on NADP. Engine out acceleration height depends on obstacles and thrust remains constant (or increased) up to clean configuration.

[172_driver]

@172_driver

I thought one of the flex requirements was that it couldn't be lower than climb thrust. Unless you are strictly speaking of derate only, in which case nevermind me.

Not that I have heard of, sorry. I was talking derate+assumed temp.

[Escape Path]

Maybe it's a specific derate thing.I can't quite remember where I heard that, but I did. Will try to look for it. I've noticed that in our flex takeoffs, with very light weights, thrust remains the same when we select climb thrust

[Capn Bloggs]

Happens on my machine when light. Nice whoomp up the rear end at 1500ft when throttles increase to climb EPR to get things going a bit quicker! :ok:

[vilas]

Escape Path

In Airbus there is a software installed which prevents thrust increase when thrust levers are brought to THRCLB from Flex. Initially the Thrust is frozen and then slowly blends into climb thrust.

[ex902]

Thank you very much for your valuable comments and discussion.

[Escape Path]

Escape Path

In Airbus there is a software installed which prevents thrust increase when thrust levers are brought to THRCLB from Flex. Initially the Thrust is frozen and then slowly blends into climb thrust.

That's helpful Vilas. Will try to catch it next time I'm on the bus