I have seen jet engines with nozzles being able to dilate to adjust for throttle and things of the sort, but the question I have is, if the engine is turned off, does the nozzle "reset" to a resting or neutral position? Or does it just stay how it was when the engine is switched off. Also, can the nozzles be moved by hand? And does the nozzles' dilation change if the aircraft is left idle for long periods of time?

Specific engines that I would like to know about are the F100-PW-100/200/220/229.


2 Answers 2


I work with the F100-PW-200/220, so specifically for these engines, i can give you an answer.

The convergent nozzle actuators are mounted to the exhaust nozzle support and consist of one primary and four secondary ballscrew, linear-type actuators.

This answers your first two questions. Since it's not directly hydraulic, the ballscrew actuators will stay in their last position if you just directly shut the engine down.

You can't actuate the nozzle by hand, unless you can somehow rotate the ballscrew actuators.

Edit: To elaborate on this, the nozzle won't return to a neutral position due to the engine switching off and losing hydraulic pressure. The Electonic Engine Control will however - as part of the shutdown procedure - leave the nozzle in a ready position

And for the last question about dilation in idle, the previous answer is correct! Depends on the pressure in the exhaust section. You don't want to accidentally choke your engine and risk a flame-out. I don't see the engine ever building up enough pressure to reach this point in idle mode though.

  • $\begingroup$ Baring failure, the engine should be in idle before shut down anyway so the nozzle should end up in the same position every time, shouldn't it? $\endgroup$
    – Jan Hudec
    Dec 17, 2021 at 17:22
  • 1
    $\begingroup$ @JanHudec The primary nozzle actuator is driven by two drive cables from the convergent exhaust nozzle control (CENC). The two cables are used to provide a fail-safe mechanism in case one cable malfunctions. The CENC is controlled by the Electronic Engine Control. With the EEC off or inoperative, the nozzle is nearly closed at throttle settings above midrange. The nozzle can be put in a mode, where the nozzle is not scheduled by the CENC but is aerodynamically loaded toward the closed position. $\endgroup$
    – Noddle
    Jan 3 at 9:31
  • $\begingroup$ So yep, during normal shutdown with no failure it will always end up in the same position $\endgroup$
    – Noddle
    Jan 3 at 9:32
  • $\begingroup$ However with EEC off or inoperative, it would land and shut down with the nozzle towards the closed position. $\endgroup$
    – Noddle
    Jan 3 at 9:33
  • 1
    $\begingroup$ I see what you mean now @JanHudec although the nozzle won't go to to a "neutral" position when shutting the engine off - the entire shutdown procedure has the engine go to 75-78% power, and then to idle. In which the Electronic Engine Control will adjust the nozzle automatically depending on the pressure in the exhaust section. This automatic adjustment is typically the same every time. $\endgroup$
    – Noddle
    Jan 4 at 7:33

The reason jet nozzles change diameter is due to efficiency. A jet (or rocket) nozzle is most efficient if the pressure of the exiting gas is equal to ambient air pressure. If the pressures aren't equal, then some thrust will be wasted. This is why rockets have large bell nozzles, to allow the high pressure exhaust gasses to expand and thus lose pressure until it matches outside air pressure as closely as possible.

Variable-geometry nozzles are most often found on afterburning engines. Using the afterburner results in an increase in exhaust gas pressure, so the nozzle needs to expand in order to allow the pressure to drop to match ambient.

Unfortunately, the answers to a lot of your questions are going to be different from engine to engine. I'm not familiar with the specific one you asked about, and a quick Google search didn't turn up anything useful. Nevertheless, I'll answer what I know.

If the engine is turned off, does the nozzle "reset" to a resting or neutral position? Or does it just stay how it was when the engine is switched off?

Since there is no point in moving the nozzle when the engine is not producing thrust, I strongly doubt that there will be any kind of resting or reset position for a given engine. That having been said, if the mechanism isn't perfectly balanced, then residual hydraulic pressure (or just gravity) may push the nozzle into either the fully open or fully closed position over time.

Can the nozzles be moved by hand?

Most such nozzles are hydraulically operated, so whether or not you can move them by hand depends on whether or not the hydraulics are pressurized. Some may be moved by an electric motor turning a jackscrew, and since reverse-driving a jackscrew is nearly impossible, the answer would be "no" in that case.

Does the nozzles' dilation change if the aircraft is left idle for long periods of time?

The diameter of the nozzle depends on the difference between the exit gas pressure of the engine (which may be measured directly or estimated from the engine's current power setting) and ambient air pressure. If neither of these are changing, then there is no reason to change the nozzle.


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