In the Aeroplane Operating Manual of DHC Q400 aircraft it is written that, at the time of normal engine shutdown, the engine system tests the NH overspeed circuitry by using it to shutdown the engine.

  1. I didn’t understand that, how the NH O/S circuit is used to shutdown the engine and how it is tested?
  2. And what are the conditions that will ensure the correct operation of NH O/S circuit?

1 Answer 1


The same statement is made here in a document describing the PW150 turboprop engine, which powers the de Haviland Canada, now Bombardier, Dash-8.

It gives a little more detail in section, NH overspeed protection.

It says:

The PW150A Powerplant has an independent overspeed (O/S) protection circuitry (dual chan- nel) built into the FADEC which has the capability to cut off the fuel flow through the Fuel Shutoff Solenoid. Independent NH signals (from the FADEC) are used by the O/S circuitry. A fuel shutoff command is issued when the measured frequency of the NH input signals exceed a pre-pro- grammed threshold value of 108%. The O/S Protection circuitry is exercised on normal shut- downs by the FADEC.

So, it would appear that, when testing the overspeed protection in a normal shutdown, they do not actually allow the shaft speed to increase to 108%, but rather they electronically alter the speed sensor frequency signal, so it appears like the shaft speed is greater than 108%, which then cuts off the fuel flow.

NH is the term used to describe the high pressure shaft speed, measured in units such RPM.

  • $\begingroup$ Thank you for the reference.. I read that paragraph just now.. At first I thought that, when the frequency of the electrical signal is 108% of it’s value then the solenoid will close fuel flow. But it is clear now that, when the NH RPM is 108% of it’s value, then it will alter the frequency and FADEC will close the fuel flow through solenoid. But how it is used to shutdown the engine normally and teated for correct operation? I mean, to test it’s operation the NH RPM will have to reach 108% of it’s value. Does it happen in real time operation? $\endgroup$ Nov 19, 2017 at 11:31
  • $\begingroup$ @Mirajul.Pias. If the aircraft is flying along, and NH exceeds 108%, the O/S protection cuts off the fuel flow. But when the O/S protection is being tested in a normal shutdown, the NH speed does NOT reach 108%. Instead, the bias the electronic speed sensor signal so it only looks like the NH speed has reached 108%. I modified the answer with the text in bold, as it might have been a little ambiguous. Hope that makes it crystal clear. $\endgroup$
    – Penguin
    Nov 19, 2017 at 21:01

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