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Reduction in Max Continuous Power in turbine helicopter with increase in altitude?

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Reduction in Max Continuous Power in turbine helicopter with increase in altitude?

Old 24th Jun 2021, 17:32
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Question Reduction in Max Continuous Power in turbine helicopter with increase in altitude?

To all our helicopter Brainiacs' out there, a question if I may ask please?

Why does the Maximum Continuous Power (OEI) available (and even 2.5 minute rating) reduce in a turbine helicopter with an increase in altitude?

Example: After an engine failure at Lafayette Regional Airport(42 ft AMSL), the AW139, MCP OEI at 20C is 140% Tq (PI).

Lets also assume Standard Adiabatic Lapse Rate of 1,98C (2C) up to 10 000ft.

1. Why at 10 000ft ASMSL (0C) with one engine failed after being in the cruise with 70% / 70% PI AEO, will the MCP OEI drop to 113% PI? (most likely ITT limit due to altitude and not TQ)

2. Will the PI Limiting also be reduced from 140% PI to a value of 113% (PI) maximum?

3. Is there a possibility of harming the remaining good engine coming of 70% PI AEO in cruise to an OEI condition in the cruise?

I can unfortunately not post URLs but if someone can help, the Graphs are in AW 139 RFM, Section 9, Fig 9-59 (OEI CRUISE - PA 0 Ft at 20C) and Fig 9-86 (OEI CRUISE (PA 10 000Ft at 0C)

Thank you.







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Old 24th Jun 2021, 19:13
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Originally Posted by GOMMY View Post
To all our helicopter Brainiacs' out there, a question if I may ask please?

Why does the Maximum Continuous Power (OEI) available (and even 2.5 minute rating) reduce in a turbine helicopter with an increase in altitude?

Example: After an engine failure at Lafayette Regional Airport(42 ft AMSL), the AW139, MCP OEI at 20C is 140% Tq (PI).

Lets also assume Standard Adiabatic Lapse Rate of 1,98C (2C) up to 10 000ft.

1. Why at 10 000ft ASMSL (0C) with one engine failed after being in the cruise with 70% / 70% PI AEO, will the MCP OEI drop to 113% PI? (most likely ITT limit due to altitude and not TQ)

2. Will the PI Limiting also be reduced from 140% PI to a value of 113% (PI) maximum?

3. Is there a possibility of harming the remaining good engine coming of 70% PI AEO in cruise to an OEI condition in the cruise?

I can unfortunately not post URLs but if someone can help, the Graphs are in AW 139 RFM, Section 9, Fig 9-59 (OEI CRUISE - PA 0 Ft at 20C) and Fig 9-86 (OEI CRUISE (PA 10 000Ft at 0C)

Thank you.
I think short answers would be:

1. Youre torque limited AEO and ITT/NG limited OEI (ITT hot and high,N1 cold and high)

2. Not sure how the AW139 FADEC works but is PI Limiting a function that works AEO and OEI. If both then your answer is yes, otherwise no.

3. No, that would have been covered in flight test.

Last edited by gipsymagpie; 25th Jun 2021 at 07:30.
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Old 24th Jun 2021, 22:14
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I should imagine that compressor speed limitations in decreasing air density may be the limiting factor. With increasing altitude, the collective pitch setting will be higher for a given airspeed than at a lower one, thus requiring more engine power to counter increased drag. OEI the engine may be unable to achieve the compression needed to match the torque produced during AEO flight at a high density altitude, without reaching a design limitation. Same thing happens with the generators.

So the answer to 2 is likely yes, you will be limited by Ng and that will dictate the PI value.

Answer to 3 is no for 2.5 mins and yes after that. The EECs will PI limit to whichever limiting factor comes first (Tq, ITT or Ng), and not allow more than the 2,5 rating to be delivered. But once that limit approaches expiry, the pilot has to reduce the power requirement.
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Old 26th Jun 2021, 22:17
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So actually air density has much less of an effect on turbines than piston aircraft. Temperature is the most critical factor. At low altitude the gearbox is limiting (usually the MAUM) then temperature kicks in (starting ambient temperature plus air has to be compressed more so gets hotter still). Then finally at really high altitude, the dominant factor becomes the sheer lack of air and the compressor maximum speed becomes the limiting factor. You can see these step changes in a typical power available chart in a military flight manual - it's slightly less obvious in the civilian presentation. OEI you are generally always temperature limited when at altitude due to having insufficient ceiling to get to the altitudes where raw air density becomes a problem.

Thank you Mr Shawn Coyle for helping me in researching that nugget with your excellent text books, may you rest in peace sir.
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