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In an answer given to How can I politely remind ATC that I'm waiting for instructions or clearance? by @Mark, he quoted part of AIM regs:

4-3-14. Communications

a. Pilots of departing aircraft should communicate with the control tower on the appropriate ground control/clearance delivery frequency prior to starting engines to receive engine start time, taxi and/or clearance information. Unless otherwise advised by the tower, remain on that frequency during taxiing and runup, then change to local control frequency when ready to request takeoff clearance.

NOTE- Pilots are encouraged to monitor the local tower frequency as soon as practical consistent with other ATC requirements.

REFERENCE- AIM, Automatic Terminal Information Service (ATIS), Paragraph 4-1-13.

b. The tower controller will consider that pilots of turbine-powered aircraft are ready for takeoff when they reach the runway or warm-up block unless advised otherwise.

I have bolded paragraph b because that is what I am asking about here. Why is the tower controller to consider turbine-powered aircraft to be "ready for takeoff when they reach the runway"?

Why are non-turbine engines (piston?) not ready? What does this paragraph cause the tower to not have to do?

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Piston aircraft require a run up, in which the pilot sets the power to a specified RPM, in order to check proper functioning of a variety of systems, and to ensure that the oil is warmed up to a specified temperature. Among the systems to be tested are carborator heat (for normally-aspirated engines), magneto operation, alternate air (for fuel-injected engines), hydraulic systems, and propeller governor (for variable-pitch props). In an aircraft with a hydraulically-activated variable pitch prop, the prop is "exercised" by varying the pitch, which allows the cold motor oil in the prop governor to mix with the warm oil in the engine sump, to bring the entire lubrication system's oil temperature up into the green arc. During the runup, the vacuum system is checked to ensure that enough vacuum pressure is provided to critical instruments. Lastly, a grounding check is made with the magnetos, to make sure that the magneto electrical system is properly grounded.

Performing a run up at the ramp would prove dangerous to people and aircraft if not tied down and due to FOD.

Generally turbine warmup occurs pretty quickly after startup and during taxi. They are usually ready to go at the ramp.

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    $\begingroup$ The important point you implied but didn't state explicitly is that turbine aircraft don't need those (nor any other) run-up checks. $\endgroup$
    – Jan Hudec
    Commented Oct 21, 2014 at 5:04
  • $\begingroup$ Good answer. Additionally, turbine powered commercial aircraft are generally larger than piston aircraft and can't just turn off into a run-up area to let the more prepared past. If you aren't ready to go by the time you get to the runway you will delay those behind you. Not a good way to make friends... $\endgroup$
    – Michael Hall
    Commented Nov 30, 2018 at 1:41
  • $\begingroup$ If the engines aren't run up until the aircraft is about to take off, what does a piston aircraft taxi with? $\endgroup$
    – Vikki
    Commented Mar 13, 2019 at 3:28
  • $\begingroup$ @Vikki If the engine fails on the ground, that's not an emergency. Thus, the plane can taxi without having to do a runup first. But an engine failure in flight is an emergency, thus you want to test the engine (and related systems) with a runup sometime before you take off. $\endgroup$
    – HiddenWindshield
    Commented Jan 7, 2022 at 15:41
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The tower controller can plan that the aircraft will be ready to line up and depart.

This is important in sequencing the traffic at busy airports. If your watching a number of aircraft taxi out to the hold, its likely (dependant on the aerodrome layout) that the piston aircraft will have a number of run up checks to do, which can take a few minutes. Whilst a jet aircraft should be ready to depart.

Certainly in commercial jet ops it's more efficient to be ready to go by the time you reach the hold. Any time spent on the ground decreases the effective utilisation of the aircraft. Controllers are normally aware how keen pilots are to get airborne.

Side note: Jet engines also require a warm up time, this is normally a minimum oil temperature which must be achieved prior to setting takeoff thrust. There is normally no special procedures, warm up being accomplished by the engines being used to taxi the aircraft.

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With today's larger turbine engines, there's no "run up" as such except in icing conditions with temperatures below about 3 degrees Celsius. Under more normal conditions, both General Electric and Pratt & Whitney engines powering today's biggest jets need about 3 minutes and 5 minutes respectively, of no more than taxi thrust levels and oil temperature off (above) the bottom mark to permit take off. On some, the procedures even mention that the take off need not be delayed on account of these running times as long as the oil pressure is registering.

There's very little else or nothing by way of engine checks before departure. Secondary engine parameters such as oil temp and pressure are normally not even displayed except during start and automatically in case the numbers start changing to a 'not normal' value.

Turbines tend to be mechanically far less complex than reciprocating engines and less prone to failure. Keeping in mind these almost invisible engine operating procedures, ATC expects them to come up to the runway in sequence and accept take of clearance without delay.

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  • $\begingroup$ Welcome to Aviation, skipper44! Thank you for your useful answer. I have edited it slightly, and please feel free to roll back my edits if you disagree with them. You might find it helpful to review the site tour and the help center. Thanks! $\endgroup$
    – CGCampbell
    Commented Oct 30, 2020 at 10:21

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