Tailrotor failures are often disastrous. Despite this the causes are little understood and most pilots are not trained to cope with them

Helicopters suffer from a weakness that is critical, under-researched and affects them more often than certification requirements or military specifications allow, according to an in-depth UK Civil Aviation Authority study. This weakness is tailrotor failure, and it matters because its effects are frequently disastrous.

Helicopter tailrotor failures are "time-critical emergencies" that can have several different causes. If the pilot does not identify the cause of failure within about 2s and react appropriately, the helicopter will become uncontrollable, according to the CAA. Its research is aimed at improving understanding of how tailrotor failure occurs, how pilots should be trained for it, and how it might be reduced or its effects mitigated.

To add to pilots' decision-making problems, a phenomenon known as loss of tailrotor effectiveness can rob a helicopter of anti-torque and directional control without any single-rotor failure.

Helicopters depend completely for their flight control on a system that counteracts the main-rotor torque to prevent the aircraft fuselage rotating. This can be a tailrotor or an alternative counter-torque arrangement like the NOTAR (no tailrotor) circulation control tailboom system. The latter is only available so far in the MD Helicopters MD 520N, MD 600N and MD 902 Explorer. Tailrotor blade failures can result from striking obstacles, from fatigue sparked by manufacturing flaws, or by a fault anywhere in the tailrotor drive train, or in the blade pitch-control mechanism.

Regulation gap

Examination of civil and military records of such failures shows the rate is "significantly worse" than the specified airworthiness targets set by the CAA and the European Joint Aviation Authorities. It is this "regulatory gap" in helicopter airworthiness, and a lack of information about the performance of each type in the event of a tailrotor failure, that prompted the CAA study. The objectives were to discover how to raise tailrotor system reliability and create failsafe strategies in the long term, and to warn pilots and operators of the risks and reactions in the meantime.

To help define the objectives of the tailrotor failure study, the CAA used Canadian, New Zealand, UK and US military and civil helicopter incident/accident statistics, and an earlier UK Ministry of Defence study that included windtunnel tests. It also contracted UK research organisation Qinetiq to carry out experimental work, much of which was performed at the company's Bedford base using its advanced flight simulator with a full visual display.

The CAA study determined that half of all failures happen when the tailrotor strikes or is struck by an object. The methods for preventing these events involve making pilots more aware of the proximity of objects to the tailrotor - perhaps via sensor systems - or partial protection of the rotor tips by a Fenestron system. One third of failures are caused by malfunction of the tailrotor drive system, the CAA has found, leaving tailrotor control failure to account for about 17%. Health and usage monitoring systems (HUMS) applied more diligently to tailrotors and their drive/control systems could - by detecting early component vibration - prevent 18% of tailrotor failures overall, says the CAA.

Recovery techniques for failures of the tailrotor drive and tailrotor control are different, and the CAA says: "Both these failures are time-critical emergencies. The pilot has to identify and diagnose the tailrotor failure type and react with a control strategy within a few seconds or less, to prevent the aircraft departing into an uncontrollable flight state. Even if the pilot recovers from the initial transients, yaw pedal control will have been lost and the ability to manoeuvre safely and carry out a safe landing will have been significantly downgraded."

Taking into account phase of flight, the largest number of tailrotor failures occur in transit (27%). Most fatalities also happen in transit (56%), according to statistics. But considering the lesser time spent in the transition phases of take-off and landing, 51% of all tailrotor failures occur in these high-torque phases compared with other times, which account for 41% of events. A review of UK and US civil and military accidents showed the rate of tailrotor failure is between 9.2 and 15.8 events per million flying hours, which "far exceeds even the military specification". Tailrotor drive failure was most likely to lead to crew fatality.

Leaving aside the improved and more extensive use of HUMS, the CAA suggests several "prevention and mitigation" measures for tailrotor failures, while admitting that most introduce penalties or other disadvantages. These include:

for tailrotor control failures, designing in a failsafe blade-pitch system "as currently used in some types of spring bias unit"; increasing fin effectiveness to counteracting tailrotor failure yaw in forward flight, possibly incorporating a deployable fin; a variable tailboom strake; retrofittable drag chute; a twin tailrotor system; a warning system that directs pilot recovery tactics; an automatic flight-control system with increased pitch/roll attitude hold authority.

The pilot's view

The problem with training pilots to respond effectively to tailrotor drive failures, says the CAA, is that this can only be properly taught in a Level C or D simulator - the most capable - with a good visual system. But there are few available, and the use of such capable devices is costly. It also cautions that generic, non-type-specific recovery techniques may be better than no advice, but they must be validated on type. At present, the CAA's inquiries found, "most training schools discuss the subject of tailrotor failures to a certain extent, and some schools demonstrate the effects of control jams. On some advanced courses, full pedal inputs during the hover are demonstrated to give some impression of yaw rate."

Professional helicopter pilot Steve Collard of CHC Scotia says he was "surprised by what we discovered" when he was working with the research team simulating forms of tailrotor failure. In his appendix to the CAA/ Qinetiq study, he observes that flight manual advice to aircrew on tailrotor failures "is on occasions poor, misleading or virtually absent". Collard's thoughts on training are revealing, as much about what is lacking in helicopter manufacturer test programmes as about what is lacking in today's training: "Training for tailrotor failures is important. For this training to be effective, a number of requirements must be in place. The guidance in the flight manual must be comprehensive, unambiguous, realistic and validated to the highest possible level."

Collard acknowledges the lack of practical simulator availability: "Training for tailrotor failures is not really possible in the aircraft, with the exception of some 'jammed pedal' procedures, and it is not normally practical to build simulators for smaller helicopters, so this flight manual advice is really the only source of advice for such aircraft. Where simulators exist, they should, of course, be used as much as possible. However, they will be of little practical use if the model on which the simulation is based is incomplete or incorrect." Simulations often give a kinder impression of the effects compared with reality, Collard adds.

Collard says he would like to see helicopters designed "with sufficient inherent yaw stiffness to contain the transient worst case response within at least structural limits." Who said being a helicopter pilot was easy?


Source: Flight International