What is bleed valve range?

[Stewy44]

Hi guys, I've done lots of searching on Google, but couldn't find the answer.

I read somewhere that for turbine jet takeoff, the brakes should be applied and the engines spooled up to a setting "beyond bleed valve range" and then allow the engines to stabilise.

I was just curious, what is bleed valve range?  And, roughly what percentage of N1 would this be for a 737, 747 and 777?

Cheers guys
Stew

[expat]

After 5 years working on the Boeing 737-800, I can say that I have never heard of this phrase. However, reading between the lines I assume that it is refering to the Bleed Cross Over. This is when the engine is producing enough thrust that the engine bleed system changes from the HP compressor to the LP compressor. This happens at approx 60% N1, but it also depends on the outside air temperature. As you can see, 60% N1 does not really have anything to do with stabalising at take off thrust. Also, the pilot will advance the throttles to around 80% and then once the aircraft is rolling advance them to the take off setting. This reduces the "slam" forces on the engine and the airframe. There is no real period of stabalising with the brakes on........unless we are doing a run up post maintenance.

Matt

[Splinter562]

Interesting that they'd refer to it that way. As you may have gathered from expat's response, this is actually a fairly advanced turbine topic. What expat's covered is the bleeds relating to the aircraft's deice and environmental control system. Those are the bleeds that go from the engine out to the airframe. As he said, there is no real stabilizing involved.

The bleed valves I believe your reference is talking about are what is typically called "acceleration valves" or "handling bleed off valves". These valves provide a bypass from certain compressor stages. Their job is manage engine stability during rapid engine accelerations. In modern engines, these valves are computer controlled and the pilot doesn't need to worry about them at all.

The concept of spooling up the engines to a given point before moving to full power is still good practice. Turbines take a long time to spool up from low RPMs. "Standing the throttles" gets the engines up to a speed where they will accelerate quickly. It allows allows the pilots a quick second to scan the engine instruments and make sure everything is in the green before starting the takeoff roll.


If you still want more info on these valves, here ya go:

Before they were computer controlled, these valves were more of a concern to pilots. The acceleration valves are a sort of give and take. With the valves open, the engine is protected from surging or flame-out during rapid engine acceleration but can't develop as much thrust. With the valves closed, performance is increased but you cannot make rapid throttle movements. For takeoff, the valves are open during the initial engine acceleration to keep the engine happy. Then the valves are closed (perhaps automatically at a certain N1) and the throttle is smoothly advanced to takeoff so that the engine can develop full power.

[Stewy44]

Guys, these are incredible answers that I found very interesting and informative, thank you so very much for your help!
Cheers
Stew