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Hughes 300 MAP question


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The S300C uses a de-rated engine. Other components are only rated to handle 190HP; at low density altitudes it is possible for the engine to produce more than 190HP.

 

The yellow band on the MP gauge ranges from 23.3-27.2"; it represents where your MP limit will fall based on DA. One of the aircraft's required placards is a table depicting your MP limit by cross-referencing temperature & pressure altitude.

 

For example:

At 2,000'PA and 20F, your limit is 24.3"MP. At 80F, it's 26.6"MP.

 

There is at least one thread on this forum discussing de-rated engines and the theory behind manifold pressure; I suggest you both read it. It's a critical part of flying piston helicopters.

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I understand the MP limit placard and I use it to find the max power I can use for a particular condition, but I still don't know why there is a yellow arc. Does this arc is only intended to show an area on the map where precaution need to be taken in relation of the DA placard, but there is no other restriction?

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Yes, You are correct. It is merely showing where max power may be based on altitude and temperature. In the earlier 269B it didn't have a variable manifold pressure placard so 26.1" was max power... on a hot day or cold day. The MP limit was established for sea level pressure 59 deg F. Obviously when it is hot outside there is a penality compared to the C model which allows you to pull up to 27.2 on that hot day.

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So there is no problem to run continously 26" in flight if the DA according to the placard permit it?

 

Since the limitation comes from the drivetrain rather than the engine it's obviously permitted. It wouldn't make any sense to put any kind of duration-limited stress on the dynamic components. There is a maximum they can take and that's that. On another note, if it was not continuously permitted it would clearly be stated in both flight manual and on the placard.

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The yellow range simply means you’re going to have a limitation (based on temp and alt) so you better know what it is. It has nothing to do with pilot reaction time if the engine fails……. This goes for the Robinson products as well…..

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Why there is a yellow arc from 21 to 27 since there is no time limit up to 26?

 

 

But I still don't know why there is a yellow arc. Does this arc is only intended to show an area on the map where precaution need to be taken in relation of the DA placard, but there is no other restriction?

 

 

So there is no problem to run continously 26" in flight if the DA according to the placard permit it?

 

It's as written in the Hand_Grenade_Pilot, Apiaguy, and Spike post above. There has been some confusion about the yellow arc markings on powerplant instruments in that they always indicate some type of time limit range. That is not always the case.

 

Instrument markings are mandated under aircraft certification. In the case of the Hughes 269 the requirements were under CAR 6. More current helicopters outline similar requirements under § 27.1549 and § 29.1549

 

The yellow arc should alert you that operation in that range is contingent on addition relevant conditions or circumstances.

 

CIVIL AIR REGULATIONS (CAR) § 6.734 Powerplant instruments; general.

 

All required powerplant instruments shall be marked in accordance with paragraphs [a] through [c] of this section.

 

a. The maximum and the minimum (if applicable) safe operation limits shall be marked with red radial lines.

 

b. The normal operating ranges shall be marked with a green arc not extending beyond the maximum and minimum safe operational limits.

 

c. The take-off and precautionary ranges shall be marked with a yellow arc.

Edited by iChris
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Thank you for the clarification iChris…

 

Okay, so I’ll rephrase…. A “marking” (in this case a yellow range), will always have a meaning which is defined somewhere. It’s up to the PIC to know what that “marking” means. Normally, it will be defined in the Flight Manual and specific to the aircraft being operated. That is, my yellow range(s) on my AS350 may have a different meaning to a yellow range on a S92. And, I wouldn’t put too much value on “corporate knowledge”…..

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