Conditions That Cause Autorotation Mishaps, Part 2

The substantial damage in nine autorotation accidents reviewed by the author was attributed to hard landings. Improper flare technique was specifically cited in seven accidents, five of which occurred during power-on recoveries. Four accidents involved loss of rotor RPM, and all of these occurred in the final moments of a landing flare.  

Every autorotational flare will be different depending on the existing wind conditions, airspeed, density altitude, the specific make and model of helicopter, and the rotorcraft gross weight. 

The “FAA Helicopter Flying Handbook” (FAA-H-8083-21A) states in part: “Care must be taken in the execution of the flare so that the cyclic control is neither moved rearward so abruptly that it causes the helicopter to climb nor moved so slowly that it does not arrest the descent, which may allow the helicopter to settle so rapidly that the tail rotor strikes the ground. [E]xtreme caution should be used to avoid an excessive nose-high and tail-low attitude below 10 ft. The helicopter must be close to the landing attitude to keep the tail rotor from contacting the surface.”  

Other errors included misjudging a proper height above the surface to begin the flare. This latter point is dependent on the pilot’s accurate perception of the visual cues of the landing environment.

This situation was illuminated in the NTSB report of a hard landing in Tampa on June 21, 2021, involving a Bell 407. Pilots were conducting night-vision-goggle (NVG) autorotations at the time and were well acquainted with the runway used for their practice. The accident pilot stated he had conducted “hundreds” of NVG autorotations to this runway in the past. However, prior to the night of the accident, the runway had recently been repaved and was much darker in color. As such, the visual cues the pilot was accustomed to had changed, and most likely affected his ability to judge the helicopter’s height and speed above the ground, which resulted in a higher-than-normal flair, low rotor rpm, and subsequent hard landing.

The flare will also differ greatly with changes in density altitude. This was especially apparent to the author during site visits to flight schools in Florida, Hawaii, Montana, and Utah to participate in recurrent training in autorotations. At higher density altitudes, with less dense air, there is less drag on the rotor blades. With the collective fully lowered, the rotor RPM will be faster at high density altitudes than at low density altitudes.  

The rotor RPM with a fully lowered collective may be high enough to exceed the power-off limitations. A slight amount of collective pitch may be needed to maintain the rotor RPM within limits. According to helicopter flight test expert Shawn Coyle, this is especially true on rotor systems with more than two rotor blades. “It may not ever be possible to fully lower the collective,” he says.

Other effects on an autorotation performed at high density altitude include a higher rate of descent, reduced rotor RPM build in autorotation, low initial rotor RPM response, the requirement for a higher flare height and reduced engine performance for the go-around. The difference in the characteristics of the autorotation were significant compared to sea level.  These last two points illustrate the importance of having an exposure to a wide variety of environmental conditions during autorotation proficiency training.

Full Touchdown Practice

The majority of in-flight training autorotations are terminated with a power recovery to a hover. The FAA Airman Certification Standards (ACS) do not require applicants for the private, commercial or ATP certificates to demonstrate proficiency in full touchdown autorotations.  Neither does 14 CFR 135 during initial and recurrent training.   However, the Flight Instructor ACS do require a CFI applicant to demonstrate proficiency in full-touchdown autorotations.  

The US Helicopter Safety Team’s (USHST) “Airmanship Bulletin: Full Touchdown Autorotation Training” highlights the pros and cons of full touchdown training. Advocates for full touchdown training believe the benefit the pilot receives in experiencing an autorotative landing and the opportunity to build proficiency in the technique greatly increases pilot confidence, thus reducing the change of a catastrophic outcome to a real engine failure. The team also believe that the power recovery aspect of the autorotation does not resemble the real situation and may even build a false sense of security on the part of the pilot.  

In comparison, advocates for power-recovery claim that the increased risk of damaging the rotorcraft with the full touchdown maneuver is not worth the benefit gained over a power recovery to the hover. They also believe that with the increased reliability of today’s modern engines, the industry would damage more rotorcraft practicing for an event that rarely occurs.  The USHST’s Airmanship Bulletin does not take either side in this debate.

With the lively debate on the pros and cons of full touchdown autorotation practice, it is worth asking about the prevalence of this maneuver within the data sample. It is perhaps surprising that there was only one accident. The NTSB attributed it to the pilot’s improper landing flare during a full-down autorotation. A lower-than-normal rotor rpm, faster-than-normal ground run speed, and the subsequent loss of directional control led to a runway excursion and roll-over.

Inadvertent Throttle Movements
On Feb. 18, 2020, near Tampa, the flight instructor in an Aerospatiale AS350 directed the student to conduct an autorotation with a 180° turn, followed by a power recovery. Abeam the departure end of runway, the instructor moved the throttle lever from the “fly” position to idle. While conducting the maneuver, the trainee overshot the runway and aligned the helicopter with the parallel taxiway.

Upon realizing that the helicopter was not in the correct position, the pilots chose to abort the maneuver and perform a go- around. While the pilot continued to fly the helicopter, the instructor inadvertently moved the throttle lever from idle aft toward the “off” position and then forward to the “fly” position. The engine experienced a total loss of power, and the instructor adjusted the throttle to no avail. The instructor took the controls, conducted a power-off autorotation, and landed the helicopter on the taxiway. The helicopter skidded about 180 ft., departed the taxiway onto adjacent grass, and came to rest in a drainage ditch.  

The NTSB determined that the flight instructor’s inadvertent throttle reduction below idle resulted in a total loss of engine power and subsequent impact with terrain following a power-off autorotation.

The FAA has issued Special Airworthiness Bulletin SW-12-12, “Conducting Engine-Failure Simulation in Helicopters with Reciprocating Engines.” The bulletin cautions owners and operators of Schweizer 269C and 269C-1 helicopters to avoid throttle chops to full idle to minimize the possibility of engine stoppage.  

Robinson Helicopter’s Safety Notice SN-38 (dated July 2003 and revised in October 2004), “Practice Autorotations Cause Many Training Accidents,” provides similar recommendations. It states: “do not roll throttle to full idle. Reduce throttle smoothly for a small visible needle split, then hold throttle firmly to override governor. Recover immediately if engine is rough or engine RPM continues to drop.”  

There are various flight conditions in which the helicopter will experience significant oscillations and the recommended action is to deliberately place the helicopter into an autorotation, we explain in Part 3 of this article.

Conditions That Cause Autorotation Mishaps, Part 1: https://aviationweek.com/business-aviation/safety-ops-regulation/condit…