Throttle Settings

[B Fraser]

Having never flown a jet aircraft (yet), I've often wondered why on t/o the throttles are opened to about 70% for a few seconds before t/o power is called. Is this to carry out a quick instrument check, make sure all is well in manual mode before selecting autothrottle or is there a physical engine performance reason such as allowing the ground /air speed to build before opening the taps ?

Apologies for the dumb question but I've often wondered why.

[BOAC]

On a 737, 40% is the normal run-up RPM, (70% if engine icing is suspected). 40% gets both large fans over the variable acceleration part of their speed range and they should then both run-up together - which helps to stop the aircraft going sideways

[B Fraser]

Thanks BOAC (Bend Over Again Christine ?). Ensuring that you don't suddenly trundle across the grass due to a massive imbalance of thrust would save some embarrassment.

I would never have guessed the real reason so I appreciate you solving the mystery. My money was on establishing a speed induced airflow through the compressor before chucking loads of fuel into the mix however t/o power always seems to be selected around 25 kts which scuppered that arguement.

[Engineer]

Mr McDonnell Douglas says:

"The pilot flying should ensure that engine spool up is evident and that positive throttle response is available then apply full power"

Hence on CF6 engine throttles are set to 60%N1 to ensure these factors are available before full power is applied.

Sounds safe and sensible to me

[BOAC]

BF - I have asked you over and over again NOT to call me Christine

As you can see, engines differ in handling. I guess the '25%' just happens to coincide with reaching full power? Modern engine control systems look after the fuel/air and surge line thingies, unlike the early motors.

[B Fraser]

I read somewhere that BOAC go that particular nickname at the height of the Profumo scandal.... Christine Keeler etc. etc.

way before my time of course