Preflight checks for turbine, piston aircraft
Question: Why don’t jets do run-ups like piston aircraft?
— Charlie Thompson, Texas
Answer: Turbine engines ( jets and turboprops) do not need the same kind of preflight checks as piston engines. The run-ups executed by pilots of piston-engine airplanes allow them to check the ignition system for the spark plugs, plus the propeller controls. Turbine engines do not have spark plugs and the propeller checks for turbo props are done during the taxi out.
Turbine engines have many advantages over piston engines; not needing to do a run-up is one of them.
Q: I’ve noticed that just before the takeoff run, the pilot raises the engine’s thrust to around half power, and then a few seconds later to takeoff thrust just as the plane starts its run. Why is there a twostage raising of engine thrust?
— Alan, United Kingdom
A: You are observant. Yes, modern jet engines are normally accelerated to an interim setting to allow any difference in acceleration times to be resolved at low thrust.
Engines have some difference in acceleration rate (the time to accelerate from idle to takeoff thrust). That difference can cause an undesirable yaw if one engine accelerates much more rapidly than the other.
The direct answer to your question is to prevent yawing caused by uneven acceleration of the engines.
Q: Just based on engine noise levels, I can tell that the power being delivered by the engines varies greatly during takeoff, cruise, descent and landing. What are the different power percentages that are being used during these flight stages?
— Steve Browne, Chicago
A: The difference in thrust output and engine rpm is not a linear relationship.
Takeoff and climb thrust are just few percentage different in rotational speed but significant in the thrust difference. Making this question more complex is that auto throttle systems keep the programmed speed by varying the thrust, so the cruise power setting is not a constant value.
Some jet engines measure their thrust output by the percentage of the rotation of the fan (the first and largest compressor in the engine). This percentage is around 94 for takeoff and 88 or so for climb.