There are many disagreements, often between knowledgeable experts, in the interpretation of performance charts. It happens at times that experts have different opinions but each is correct because of their individual experiences. Each test pilot who responds to certification questions does so from their individual experience. The Limiting Height – Speed Envelope (H-V or ‘Dead man’s Curve) has generated more than its fair share of controversy over the years.
My H-V testing experience has been in the testing of helicopters for US Military and FAA certification on land and sea. Most of my comments will be based on the FAA certification basis for single engine helicopters below six thousand pounds.
Certification regulations change over time and the methods and procedures to show compliance with these regulations also change. CAR6, FAR 27 or FAR 29 is the various regulations that have been the certification basis of most US manufactured helicopter s. Each individual regulation has been amended so to speak of the individual requirements of one regulation you must be specific as to time. Strange as it may seem there are different and acceptable methods of testing to show compliance with the regulations as long is it meets provides for an equivilent level of safety. Each manufacturer has individual methods of showing compliance. Sikorsky is different than Bell which is different than Hughes which was different than Robinson or McDonnell Douglas or Hiller or Boeing and every other company in the business.
The FAA published an advisory circular 27-1 to assist certification applicants by providing a history of the certification process and to provide at least one acceptable method to demonstrate compliance with Part 27 and to some extent the other parts.
The above is written so that it is understood that my experience is different than that of Nick or Shawn or other test pilots, and what follows will reflect that bias.
Nick writes:
“1) The H-V Curve is only precise at the one tested condition of weight, altitude, wind and temperature, it can be very very inaccurate if you are at a much lower weight, higher wind or much lower altitude/temp.”
I respond:
My experience is that there is a build up process during Limiting Height – Speed Envelope testing. Tests are conducted and test data generated at several different combinations of weight, altitude, wind and temperature and that data is accurate. The Limiting Height –Speed Envelope information presented in the Performance Section of the Rotorcraft Flight Manual is that required by regulation. The guidance is that the flight manual should list any procedures which may apply to specific points (e.g., high speed points) and test conditions, such as runway surface, wave height for amphibious tests, marginal areas of controllability or landing gear response, etc. The HV curve should be presented in the RFM using actual altitude above ground level and indicated airspeed. Manufacturers may include data for other tested conditions but it will be in the unapproved section of the flight manual if not validated by the FAA. The curve is not ‘inaccurate’ at other weight, winds, altitude, temperatures, landing surfaces or any other specific condition not tested; it is just not applicable to those conditions and can be used to predict performance.
Nick writes:
“2) Is based upon programmed delays in lowering the collective. If you are quick as a bunny, your personal H-V ciurve would be much smaller, if you are slower, you cannot trust the H-V curve because it is too small for you.”
I respond:
Item 2 is the reason I have chosen to post on this matter. The FAA has two primary methods of entry for H-V testing. A one second delay or normal pilot reaction time are the ‘programmed delays’ allowed by the FAA and which technique is used depends on the point tested. The technique applies to collective and the interpretation concerning allowable movement of the other controls such as cyclic and pedals, as well as the definition of normal pilot reaction time are the subject of considerable debate. Of one thing I have no doubt. A test pilot, who has completed a buildup program, is concentrating on only one task, and who initiates a simulated engine failure will be faster than a pilot who is task saturated, not practiced, and unaware of an engine failure event. No matter how fast a ‘normal pilot’ - the bunny pilot - he or she will not be faster than the test pilot who established the point on the curve. When I hear someone say that a good, experienced pilot can somehow beat the curve my blood boils. Do not count on it! As one example of the reason I am so confident in this opinion consider that scientific testing in controlled circumstances has concluded that recognition and response times can be as much as 4.4 to 5.7 seconds; contrast that with the 1 second (at most) delay in testing.
Nick writes:
3) If you are descending, the H-V curve is wrong, the real one would be very much smaller.”
I respond:
The H-V curve is not wrong. It is accurate for the stated conditions. The tested data presented in the H-V curve does not represent a specific condition of descent. You may consider the values presented for hover or climb or level flight to be conservative compared to descending flight, but not ‘wrong’.
Nick writes:
“4) If you are climbing, the H-V curve would be very much bigger.”
I respond:
This depends on the phase of flight where the climb occurs. In the take-off corridor the data reflects climb. A climbing OGE hover would however require a larger H-V curve. It is not the climb or climb rate that is the significant variable but the power used (main rotor blade pitch) because that increases rotor decay. There is also the type of engine failure that occurs. As an example a test pilot who retards throttle or power lever on a FADEC equipped engine will see a programmed rate of engine decay that would be much slower than that experienced in an actual engine failure. That equates to less time for the pilot response.
Nick writes:
5) If you have 5 knots of steady wind, the H-V curve would be very very much smaller. With 20 knots of wind, there is practically no H-V curve.
I respond:
This should be self evident by the curve itself and general pilot knowledge of aerodynamics. The reverse is true of tail winds and cross winds which is why the conditions on which the curve is predicated must be understood by the professional pilot. I have longed believed that CAT A performance should be predicated on 17 knots of down wind (hover controllability) because so few pilots understand what a tail wind can do to single engine fly away capability.
I do not disagree that the Limiting Height – Speed Envelope is a performance chart that should be used as a guide from which to make predictions based on specific conditions but; it is a useful guide nonetheless.
There are many who have long advocated a change to the certification basis for engine out performance and the Limiting Height – Speed Envelope such as more realistic pilot reaction times or additional performance testing such as a take-off power climb at VY, and actual engine out testing (Hiller did it once).