Tail-boom strikes
November 4, 2010 by Tim McAdamsSome helicopters, like the Robinson R66, have tall masts putting the rotor system high above the tail boom; others, like the MD500, have a more compact design. Engineers take into account flight characteristics of a design when considering the distance between the tail boom and the rotor disk. Even so, tail-boom strikes can and do happen.
One of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down too nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract it. This action in combination with the fact that the blades are still moving downward can result in the blades contacting the tail boom. An in-flight entry to an autorotation will also cause a nose-down pitch because of the advancing blade seeing a greater reduction in lift than the retreating blade. A pilot who overreacts with sudden aft cyclic will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades to come in contact with the tail boom. In-flight blade-to-tail boom strikes are normally fatal.
Strong wind gusts can also create a problem. A tail-boom strike can happen as rotor rpm gets lower and the centrifugal force holding the rotor stiff drops. A helicopter that is starting up or shutting down in high winds or near another hovering helicopter is particularly vulnerable. Manufacturers have used droop stops in teetering rotor systems to support the blades at slow speeds. One design uses spring-loaded droop stops with weights that pull them out of the way when the rotor speed–and related centrifugal force–gets high enough. Even so, many manufacturers and operators have maximum wind speeds for start up and shut down.
November 7th, 2010 at 11:01 pm
For that reason too I’ve heard people say that in strong winds it is best to park nose out of wind to minimise the flap down over the boom during shutdown.
November 12th, 2010 at 1:03 pm
As a retired Army instructor pilot and maintenance test pilot I have had the distinct pleasure to fly the Hughes 300, UH-1H Huey, OH-58 A/C/D Kiowa as well as the AH-1 Mod/Prd/Ecas Cobra and AH-64 Apachee in all types of weather, altitudes, weights and configurations. The most important aspects in the prevention of rotor/tailboom strikes is 1st – knowing the aircraft your flying and it’s limitations; 2nd Know and Practice the emergency procedures for your aircraft every chance you get. There is NO Substitute for hands on practice experience and the knowledge that goes along with. This is especially true in either a simulated or actual engine failure where control inputs must be coordinated during the “stabilization” (rotor RPM), “initial” (collective) and “cusion” (collective) phases of the landing process. As stated by Mick above, being aware of the current environmental factors (winds) that can effect the start and shut down operations (slow rotor RMP) of the aircraft will keep the grief monster from knocking down your door.
November 13th, 2010 at 11:22 pm
As a retired Air Force helicopter pilot/flight instructor/examiner, one of the adages I beat into my students was “3S… slow, smooth, and small” when it comes to all control inputs and manuevers. NEVER make a sudden control input and, with experience over time, NEVER overcontrol. This adage seems to have served my students and fellow flyers well over the years – and still serves me as I fly GA fixed-wing aircraft.
November 29th, 2010 at 6:51 pm
All valid comments, yet no amount of technique can make up for inherent design limitations, particularly as regards semi-rigid rotor systems. A well designed modern rotor system (such as on the AS-350 series) will always have much higher wind limitations for start-up and shutdown than any two-bladed system, regardless of technique.