Risks and Danger for Helicopter Pilots: Mast Bumping

by Manfred Harder, Royal Canadian Air Force (RCAF) (retired)

This is the second of a series of articles on the typical dangers for helicopter pilots. In this series, we will explore the risks involved with takeoff and landing, weather, approaches and night flying.

Mast bumping

This is one of those topics that causes student pilots or long-seasoned bush operators to start wondering if their wills are up to date: another one of the many misunderstood things in aviation that we need to understand to fly safely but should not be afraid of.

Mast bumping is a concern to those of you flying a teetering rotor head: those that are supported under an articulated, rigid or some other design rotor head read along anyway. Teetering rotor hubs vary amongst differing ships, in size or design. All have a teetering hinge. Robinson adds a coning hinge, whereas Bell achieves coning solely through bending of the blades. The odds of being under a teetering head are very good, since the Robinson R22, 44 and 66 are all included, as well as what may be the most recognized light helicopter—the Bell 206—in its various iterations, and now followed by its offspring, the 505. For those with bigger blades, there is the venerable Huey/Iroquois, and its lethal cousin, the Cobra. So as not to leave out the senior crowd, there is the Bell 47, and there may be others as well. That is a lot of helicopters; you have probably flown one or might fly one in the future.

Figure 1:"NTSB" shows the failure mode of the mast just below where the static stop would be touching. The mast bumping occurred with this Bell 206 rotor from the NTSB WPR19FA109 report aircraft.

Figure 2: "B206 head" is the underside of a normal Bell 206 rotor head. Finger at the bolted static stop and next to the interference area between the stop and the mast.

 

How did we learn of this unique problem? Well, time to put on your jungle fatigues and OD green flight helmet. The Vietnam War was well underway, and the multitude of aircraft types engaged was staggering, but what does everyone associate with that conflict, the workhorse UH-1/Huey? Every GI’s favourite sound was that thumping rotor whop to take them away from the hell of war. Things, at first, seemed to be going well in using this new form of troop and supply transport, MEDEVAC and even in attack roles. Unfortunately, losses began increasing and not always to the usual and expected enemy action, but from unexplained rotor system failures! Crash investigations found the rotor head was often some distance from the airframe; the mast had been sheared off! Was there a design failure, or did the 20ish-year-old pilots find a new way to overstress the machine and kill everyone in the process? With the US Army being the size it is and having the resources and expertise to carefully investigate accidents, yet there were some 7,000+ UH1s that flew over 10 million hours in Vietnam, and almost half of them were destroyed—a daunting task for sure!

So, what did the US Army with the help of Bell, the helicopter manufacturer, discover? The aircraft had an unloaded rotor system in several scenarios: 1) an attempt to maintain nap of the earth (NOE) during low G pushovers, 2) poorly coordinated wingovers and 3) turbulence.

The reason this situation becomes dangerous is that the tail rotor thrust, which is above the center of gravity, causes a roll to the right in the low G condition. Left cyclic input does not yield much roll correction, because the main rotor system is producing little thrust. The pilot applies increasing amounts of cyclic to right their ship, only to have the main rotor hub static stop, violently contact the mast first on one side and then 180 degrees later on the other side of the mast, usually causing a failure of the mast and separation of the hub from the aircraft (the bumping you may feel during sloped landings is a mild cousin to the low G scenario, but any contact of the stop to the mast is undesirable). Helicopters are especially dangerous to transitioning airplane pilots that use the stick instead of the collective to change vertical direction.

Figure 3: "Debris field" shows that the main rotor blade has separated from the NTSB WPR19FA109 Bell 206 and is some distance from the crash site. The aircraft was not complete upon impact. This was similar to the UH1 crashes in Vietnam that alerted the US Army to the mast bumping problem.

The correct technique to recover from this low G right roll is to reload the disk with some gentle aft cyclic. Then, when roll control is established through positive rotor system lift and hence positive G, apply gentle left cyclic. Of course, the real lesson to be learned is to avoid situations that cause low G (no cyclic pushovers, if entering areas of turbulence slow down, and finally, leave wingovers to the military). It is worth noting that since the onset of the awareness and training of mast bumping, Bell has not seen this failure in the recallable past.

Robinson helicopters had some similar mishaps and commissioned a study to see if their rotor system was inherently at risk to mast bumping. The studies showed that it is equally as safe as other teetering designs, if flown correctly. The Federal Aviation Administration (FAA), however, mandated some updated training syllabuses under SFAR 73 to make pilots more aware of dangers of low-G flight, amongst other topics. It did stress to never use a cyclic pushover, to use collective to initiate descents and to slow in turbulent conditions.

An interesting difference to the Bell situation is that the tail rotor on the Robinson is not as displaced from the center of gravity; therefore, it does not cause as much right roll as on the tall UH1 tail during low G. The Robinson models however have a single sided horizontal stabilizer on the right side of the tail boom opposite the tail rotor. This stabilizer with its downward force causes a similar rolling moment in low-G flight to the UH1 situation. The similar recovery technique will mitigate this problem: gentle aft cyclic to load the disk then correct the roll with left cyclic.

Robinson went a step further in its goal of continued safety improvement, including that outside of the normal flight envelope, all new Robinson helicopters have a double-sided horizontal stabilizer mounted on the tail boom forward of the tail rotor, which reduces the rolling effect in low G. Robinson has made this design available as a retrofit to earlier aircraft.

If you fly an aircraft with a teetering head, or even if you don’t, reacquaint yourself regularly with your AFM. Discuss various situations with an instructor or check pilot, and fly with them. They often have safe and appropriate techniques to stimulate the learning process.

References

Take Five… for Safety—Passenger Safety Briefings: Why, When and How

Review what to include in passenger safety briefings and how to address common issues than can arise

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