Increasing speed above 10,000 ft

[Nicholas49]

Can I ask a question about engine thrust on jet aircraft?

If I understand correctly, commercial jet aircraft are restricted to fly at a maximum speed of 250 kts below 10,000 ft. I also understand that 10,000 ft is the typical altitude at which the Commander switches off the passenger seat-belt sign, weather permitting of course. I am assuming, too, that above 10,000 ft, you will want to accelerate the aircraft as you make your way up to your cruising altitude / speed.

Now, this said, please could someone explain why at 10,000 ft, when the seat belt signs go off, there is (in my recent experience on a B737 and an A320, at least) no detectable audible increase in engine thrust in the cabin above the sharp increase in power on take-off? How slight/great is the typical increase in % N1 above 10,000ft?

Thanks for your help!

[1pudding1]

Typically, zero. Put the nose down a bit and accelerate.

[Nicholas49]

Ah, OK. So the acceleration is achieved simply by reducing the pitch?

[Wireless]

You maybe could hear a thrust increase in the case you mention if the aircraft was climbing out of one of the busier airports with large amounts of aircraft closely above, so of course it's not really cricket to climb as fast as possible. In this instance we would set the aircraft to fly at a particular lower rate of climb. With 250kts as a target speed also set the only way the aeroplane can now achieve both these requests is by climbing at reduced thrust. Now of course when we pass 10,000ft and select a higher speed the thrust will increase to speed the thing up. Ideally though in nice clear airspace you just climb out quickly, get to 10k and as the man says, lower the nose to speed up.

[SNS3Guppy]

Nicholas,

Generally we set climb thrust as part of the noise abatement departure procedure. Between 1,000 and 1,500' above field elevation, climb thrust is set. We clean up at 3,000' above the departure elevation (different operators may use different profiles), continuing with the climb thrust. That thrust is maintained through the climb below, and above 10,000'. During light operations, a slightly reduced climb thrust may be used.

At 10,000', we reduce the rate of climb, and begin accelerating. As the airplane approaches the desired climb speed, the rate of climb is allowed to increase, and we maintain the climb speed. Upon reaching about 27,000 to 29,000', we begin reducing airspeed to maintain a mach number, and hold that mach number, along with climb thrust, as we continue climbing to the initial cruise altitude.

Level-offs may occur on the way to the cruise altitude. Power changes will occur when leveling off.

[aviatorhi]

The answer is... it varies.

There are cases where you can have the speed restriction deleted in the first place, when going "oceanic" (out of the US anyway) once you cross the 12 mile limit, regardless of altitude you're free to go full speed ahead (depending on traffic you might want to coordinate this with ATC). Ideally we would climb at our Opt Climb speed as soon as possible. Crossing 10,000 I wouldn't expect a thrust increase though, as soon as you're clean you want to go to climb thrust and use pitch to control your speed.

[Northbeach]

Now, this said, please could someone explain why at 10,000 ft, when the seat belt signs go off, there is (in my recent experience on a B737 and an A320, at least) no detectable audible increase in engine thrust in the cabin above the sharp increase in power on take-off? How slight/great is the typical increase in % N1 above 10,000ft?

Thanks for your help!

Case 1; climbing through 5,000’ to 15,000”. The engines are already at climb power, passing through 10,000’ we would lower the pitch, just as you said, and allow the jet to accelerate to the target (most efficient given conditions, or ATC assigned speed). As you so well put it:

no detectable audible increase in engine thrust in the cabin

Case 2; being told by ATC to level off at 10,000’ and maintain 250 knots (or a lower altitude), and then later receiving a climb to 15,000’ with no speed restriction. While the jet is climbing and the engines at climb power the indicated airspeed at 250 is held constant by adjusting pitch. When the jet levels off (no longer needing to climb), without a reduction in engine power, the airspeed will increase.

Therefore to maintain the assigned speed, the engine thrust is reduced. The lighter the jet and more powerful the engines the more engine thrust needs to be reduced to maintain the 250 knots. The amount of N1 increase (thrust), related noise and acceleration forces experienced accelerating through 10,000’ during the subsequent climb will depend on how much reserve thrust is available (above that what was required to maintain level flight).

If you were to take a very light A380 or B747 and level off at 10,000' and 250 knots the power change would be significant both in reduction to maintain the 250 knots and in acceleration for the high speed climb. On the other hand my heavily loaded B737 the power changes would not be so noticable. The mighty A380 or B747 lightly loaded with 4 engines would have a vastly superior power to weight ratio than my Boeing 737.

[Northbeach]

Since you are interested here is some more information.

On some models of the Boeing 737 we have the option of specifically selecting the thrust level of the engine for takeoff. Other jets have the same capability although they will call it different things. In simple terms, you could think of it as selecting one of three different engines depending on how much thrust you want to generate on the takeoff. Lower thrust levels produce lower pressures, temperatures and wear on engine components prolonging engine life and increasing reliability. With an entry in the flight management computer (FMC) we tell the engines to develop high, medium or lower power for takeoff, and a lesser extent climb.

If we have cool temperatures, a long runway with no obstacles and relatively light loads there is no reason to push the engines to their maximum designed limits; we simply do not need that much performance. Under those favorable circumstances we would tell the engine to develop 22,000 lbs of thrust on this single takeoff rather than 27,000 lbs of thrust it is capable of producing.

Furthermore it may be possible to again reduce the thrust by artificially adjusting the sensed outside temperature (for engine performance parameters). We refer to this as “assumed temperature” where we enter in a calculated outside temperature value. On the MD80 a similar reduction was called “flex”.

This will further reduce the internal pressures, temperatures and speeds of the engine. Note: these reductions are only done when excess engine and aircraft performance is available because of a combination of advantageous weight, temperature, runway length, obstacles and other parameters exist.

Under the most favorable conditions we would select the least amount of thrust available from the selectable power settings, and then enter the highest available artificially inflated temperature (50C). This combination minimizes the thrust the engines must produce to what is necessary to comply with all of the regulatory performance and related safety requirements.

[Nicholas49]

Thanks for the replies!

Nick