On the only jet engined aircraft I'm trained on, the Bell 206B (Allison 250 engine), I was taught to move the throttle slowly and smoothly, especially when opening. I've also see this done with Rolls Royce testing RB211s and early Trents.

I've realised I don't actually know why this is. Is it to prevent possible flame out with a sudden increase in fuel with no corresponding increase in air mass since the engine will spool up much more slowly?

Do modern engines need this handling? In this video at about 0:17, the pilot commences the roll by more or less slamming the throttle open. My guess is that like all other computer controlled controls, the FADEC interprets this as "as much fuel as possible without flaming out".

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    $\begingroup$ It's worth noting that moving the throttle smoothly is not exclusive to jet engines: there are reasons you want to move the throttle smoothly in piston aircraft too (also here for some more general "be kind to your engine" reasons). The results on jets just tend to be more immediate and dramatic as casey noted. $\endgroup$
    – voretaq7
    Jul 8 '15 at 18:19
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    $\begingroup$ Yeah, it's pointless to slam the throttle to the stops in a tenth of a second when it could take 5 or more seconds to go from flight idle to TO power. As I was taught at racing school, the car can't accelerate as fast as you can move your gas pedal foot so just chill out. $\endgroup$ Jul 8 '15 at 20:31
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    $\begingroup$ If you're Boeing (working on the first 747), sometimes you do. If Pratt and Whitney aren't taking your flame-outs seriously, you take their president up, blow two engines and then ask if they want to go for broke? $\endgroup$
    – Mazura
    Jul 9 '15 at 0:00
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    $\begingroup$ @Mazura The video in your link appears to have been removed due to a copyright claim. $\endgroup$
    – JAB
    Oct 24 '16 at 16:08
  • $\begingroup$ The video was: Smithsonian Channel 747 The Jumbo Revolution $\endgroup$
    – Mazura
    Oct 24 '16 at 17:49

Compressor stall is only one reason. Casey is right: Moving the throttle too quickly leads to an oversupply of fuel which in turn will add too much energy to the air in the combustor, heat it and let it expand more quickly than what the turbine will accept. This will increase the pressure in the combustor over that in the last stage of the compressor, and the heated air will escape the combustor in both directions. That is a fancy way of saying that the compressor stalls.

However, the oversupply of fuel in the combustor will also overheat the engine components. Given that jet combustors run rather lean, there would be plenty of oxygen left to burn the extra fuel. Temperature would rise quickly before the air mass flow can help to cool the combustor. Both the combustor and the turbine would overheat, and in the worst case the engine would be destroyed. Modern engines use computer control to increase fuel supply only slowly. Before that, the pilot had to be careful not to move the thrust levers forward from idle too quickly.

An added risk on multi-spool engines is compressor surge from a dynamic imbalance. The lower inertia of the high-pressure spool allows it to speed ahead of the low-pressure spool, and now the later stages of the compressor run too quickly and are starved of air, because the low-pressure part cannot pump enough air to keep the high-pressure compressor from stalling. Only careful application of thrust commands or computer control can limit the extra heat in the turbine such that the high-pressure compressor will stay within its surge limits.

  • $\begingroup$ This makes complete sense, and also explains why during the start procedure on the Allinson, you watch the TOT like a hawk. If the limits are busted, you chop the fuel and continue cranking. $\endgroup$
    – Simon
    Jul 9 '15 at 11:59
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    $\begingroup$ Shouldn't the FADEC prevent this? $\endgroup$
    – rbp
    Jul 9 '15 at 17:19
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    $\begingroup$ @rbp - Where one exists, yes; the FADEC essentially serves as a "fly-by-wire" control for the jet engine. However, not all jets have them, though practically all commercial airliners currently flying do have one as it eliminates the flight engineer requirement. $\endgroup$
    – KeithS
    Jul 9 '15 at 18:39
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    $\begingroup$ @rbp I don't know if any Jet Rangers have FADECS, but the ones I flew did not. The FADEC was the slab of wet meat holding the throttle. $\endgroup$
    – Simon
    Jul 9 '15 at 19:10
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    $\begingroup$ Yes for engines without FADECs, the FADEC am won't do anything. Neither will the mixture, alternate air, or carb heat work if those are not installed. $\endgroup$
    – rbp
    Jul 9 '15 at 21:39

The issue is the possibility of a compressor stall. The combustion will directly respond to changes in the thrust lever but it takes time for this to spool up the turbine that is connected to the compressor. If the incoming air from the compressor is not of adequate pressure because it hasn't spooled up enough you risk a compressor surge. In these engines you need to slowly increase thrust so that the turbine can spool up (and hence the compressor) so the airflow through the engine remains adequate for the engine power.

Modern engines with FADEC systems should not have these issues as the FADEC can enforce rate limits for fuel flow increases or directly calculate the maximum safe fuel flow from observed engine parameters. Modern engines might also have more sophisticated compressors with variable stators that can help mitigate compressor stalls/surges.

  • $\begingroup$ I'm torn. I accepted your answer but I must say that Peter's is I think more complete. $\endgroup$
    – Simon
    Jul 9 '15 at 12:00
  • $\begingroup$ @Simon: A) Accepting the first answer, especially around here, isn't always the best policy, and B) you can uncheck Casey's answer and select Peter's if you feel it helps you more - SE is about the answer that best helps the questioner, mythical, magical unicorn-based Internet points shouldn't factor into the check-mark decision. (Nothing personal, of course, Casey - great answer!) $\endgroup$
    – FreeMan
    Jul 9 '15 at 14:23
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    $\begingroup$ @Simon accept whichever answer you feel is most useful to you and solves your problem best. Neither of us is going to fret over 15 rep, so don't worry about it. $\endgroup$
    – casey
    Jul 9 '15 at 14:24
  • $\begingroup$ Variable stators aren't a new thing; GE was using them all the way back in the 1950s. $\endgroup$
    – Vikki
    Jan 12 '19 at 23:55

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