Multi Engine Helicopters - Height/Velocity Avoid Curve

[tbc]

Section 5 (Performance Data) of a typical (lets use the EC 135) RFM gives data on the Critical Height-Velocity envelope. It allows calculation of an 'avoid' area which is critical for helicopter operation in the event of a single engine failure during take-off, landing or other operations near the ground.

The area is a combination of height (agl) and IAS as a function of gross mass, PA and OAT.

As an ex Single Engine QHI of some experience I can cope with this as well as getting my 'bloggs' to grasp it too, in the knowledge that if the engine fails within the avoid curve the likelyhood is that I/he/she will crash due to a lack of height (time) or speed to do something with an autorotation.

Can someone advise the applicability of such an avoid curve in a ME Helicopter that is operating to CAT A - Class 1 performance standards?

Am I close is suggesting that this data is provided for either:

Non-Class A operations - such as the military where they load and lift so to speak where losing one engine in this regime could hurt, despite the other one doing the business as best it can, (not a convincing argument but you never know!!)

or

Is to advise a pilot who ends up OEI that he should avoid or at least minimise exposure time on landing (methinks) as he reverts back to the understandable use of the Avoid Curve for SE helicopters.

Any thoughts/advise/guidance etc.??

[212man]

If you were to post this on 'rotorheads' I think you'll get loads of answers; some of which may be correct!

To answer the basic questions:

1. The curve is in the FLM because it has to be according to certification requirements (see following extract from FAR Part 27)

"27.1587 Performance information.

(a) The rotorcraft must be furnished with the following information, determined in accordance with §§27.51 through 27.79 and 27.143(c):

(1) Enough information to determine the limiting height-speed envelope."

2. There is no strict relevance to operations in PC1 or to Cat A standards, as the procedures developed for such operations specifically avoid the HV curve and are detailed in a separate FLM supplement.

3. Non Commercial Air Transport operators can operate to whatever PC standard they wish, but the FLM part 5 will normally describe the Cat B take off as being required to avoid the HV curve. Naturally some knowledge of this is useful for u/s load ops or camera work etc.

4. The normal, single graph, HV curve is only relevant for a specific Density Altitude and weight, and for level flight, though some aircraft types have a series of graphs available (such as the B-212 FLM) where more detail can be obtained. Normally during landing the HV will shrink and may even disappear. At some lower weights it will disappear too, hence the use of vertical Cat a procedures.

As I say, someone like Shawn Coyle or Nick Lappos will be able to expand on the subject.

cheers

[tbc]

Thanks for that - my first port of call was here to see if I could attract a more knowledgable responce like yours, but I will now open it up to rotorheads.