About OAT

[HeliEagle]

Hi
Many Flight Manual said the Ambient Teperature is around -40 ¡ãc to +50¡ãc.
Do you have any experience to break it.
Mine is over 50¡ãc during a platform take-off in exhaust gas of torch following engines fluctuating. Bad!

[NickLappos]

Those temperature limits are set by the tests that were done for certification, from subsystems exposed to ultra-cold and ultra-hot chambers, to the entire aircraft exposed to the ambient temperature and "cold-soaked" for many hours.

Generally, the high temp is set by the ability of the systems to cool themselves, especially the hydraulics and engine compartment components and sometimes (rarely) electrical components. High temps are typically derived by projecting the temperature in the closed compartments as they soak up to max temp (typically a zero wind hover). The engine compartment is often where the hottest places are.

For cold, the worst items are almost always the hydraulic servos, which shring so that larger gaps allow big leaks until the fluid and the cylinders warm up.

When you hover with a downwind and reingest your exhaust, the local heating can be pretty fierce, some helos have problems where the exhaust impinges on the fuselage, and literally burns the paint or over heats the composite structures. Avoid those downwind hovers, by all means. You could be overheating any components that rely on cooling air. Engine compartments on many turbine helos are cooled by having an ejector effect, where the exhaust pulls compartment air along with it, thus ventilating the compartment. If you hover for prolonged periods in a downwind, you can force the ejector to have nil effect, or even entrain exhaust into the engine compartment, with dire results, even fires!

Generally, the low and high temp limits are real, and damage or even unsafe conditions can occur due to abuse.

[Geoffersincornwall]

The temperature envelope is there to inform you that safe operations have been proven within those limits and providing you obey any caveats written into the Flight Manual. Routine operations outside these limits may be possible but be careful, you may find that your insurance company will take a view on whether or not their cover extends to such circumstances. Also, if you are trying to be professional about the operation and verify by prediction (using Flight Manual data) that you can achieve any particular performance level (Cat B, Cat A etc) then you will find that no data is available outside the temperature envelope. Extrapolation is not allowed so once again you are stuffed!
I can identify with your dilemma having operated a 365n with data only available up to 40 deg C. What to do when halfway through the flight you find the afternoon temp climbing to 45?

If I interpret your particular dilemma correctly you are talking about taking off from a platform where the flare is producing an increase in the air temperature over the helideck. In these circumstances the received wisdom is to limit the helicopter weight when the wind is in the 'bad' sector. You are not alone. Such problems occur everywhere in the offshore world. It's like making an approach in North Sea temperatures and arriving over the deck to find the climate there is very tropical!

In the UK the operators have cooperated to produce a "Helideck Landing Limitations" guidebook that tabulates every helideck in use in the North Sea and around the UK (platforms, drill-rigs, drill-ships and vessels) to advise the pilots an the restrictions to be imposed on any particular helideck.

More info available from the British Helicopter Advisory Board. see www.bhab.org

[MightyGem]

Ours is limited to +50 Deg C(ISA +39).
Can someone explain the ISA bit. I would have thought ISA, at 15 deg, + 39=44.

[C4]

Hey MightyGem,
I would suggest you invest in a calculator...
15 + 39 would be 54

[NickLappos]

MightyGem,

The temp is limited to the lower of the two values, either 50 C or ISA + 39 degrees. As you climb up in altitude, the Temp limit drops. When ISA + 39 is less than 50, you then operate to the ISA + 39 limit. Thus, at 2000 ft, your limit is 50 degrees, and it reduces by 2 degrees for every thousand above that. For example, at 10,000 ft, the limit is +34 C (ISA is -5 add 39 = +34)

The reason why the max temp drops with altitude is that all the cooling loses efficiency with the air density, so that the max temp must fall to keep up with the reduced cooling capability.

[MightyGem]

Hey MightyGem, I would suggest you invest in a calculator...
15 + 39 would be 54
Typo, typo!!

Thanks Nick.