Originally Posted by
Self loading bear
OK, well spotted. But there may be something else in this accident report. Something is puzzling me. In many MD600 accidents we see evidence of tail boom strikes. This was also the case for Airframe #54.
Isn’t the MD meant to be one of the fastest, most maneuverable, precise and safest (as long as the donk is running) helicopters? Or is this privilege reserved to the 500. Despite the 500 and 600 looking so similar, are they possibly two very different kind of aircraft? And is the difference maybe beyond the obvious (NOTAR vs conventional TR) and has to do with their very different proportions?
Here we go on lambasting 2-bladed teetering head choppers as being inherently prone to catastrophic mast bumping if flown ham-fistedly, or in conditions of strong turbulence. On the other side, forum members credibly report having
flown the 2-bladed JetRanger for thousands of hours in often extreme turbulence, without even the slightest on-set of mast bumping.
Only to find that
the 6-bladed, fully articulated MD-600 carries a safety warning in its Rotorcraft Flight Manual similar to the one we have for Robisons (almost identical wording to R44 Safety Notice SN-10). Here is the wording from the MD flight manual:
Normal Procedures Section 4-9: “Rotor ‘blowback": …condition is exacerbated by … low rotor RPM, high collective angle pitch setting which causes blade stall over a large portion of the main rotor disk. If the resulting blowback is excessive, the main rotor may contact the tail boom.
While the MD600 safety note is uttered in the context to running landings, the same apparently can easily happen in mid-air, under low G conditions, as was found it to be the case at least in this accident. I translate from the French accident report:
2.1 Simulated engine failure at cruise speed
At the time of the accident, the helicopter was flying at a ground speed of close to 140 KTS. At least 4 of the 6 rotor blades deeply cut into the tail boom. This damage is indicative of rather abrupt control movements. It also appears that [at the time of the tail boom strike] the rotor blades were not subjected to high centrifugal forces, in other words, there is indication of reduced rotor RPM. While such loss of rotor RPM could have been caused by loss of engine power or failure of the override clutch, no pre-existing mechanical problem was found.
The mishap flight can be classified as advanced instruction towards type rating, combined with a private cross-country flight. The instructor had 10,000 HRS on all types of Airbus helicopters incl twins, and on Robinsons, but was himself type-rated on the MD600 only 30 days before the mishap flight. He only had 32 HRS on-type (and had never flown the MD500).
The owner-student had around 1,000 HRS TTRW on R44 and EC120. The mishap flight was part of his type-rating on the MD600. The report found that the instructor tried to challenge (well, the actual word in the BEA accident report is “entertain”) the student with a surprise simulated engine failure at fast cruise and 800’ AGL.
Turbulent conditions [which according to the BEA report may have prevailed at the time of the incident] and/or the surprise element have probably triggered the incorrect response by the student pilot. The need to at the same time turn into the wind [in preparation for an autorotation landing] would have further increased the student pilot’s workload.
3.2 Accident cause
In all probability, the inappropriate control inputs of the student-pilot have caused an in-flight break-up of the tail boom, caused by the main rotor striking the tail boom.
“1.18.1 Simulated engine failures
“Entering autorotation at cruise speed is complicated. The pilot has to make small but precise control inputs to reduce speed to Vy. Dumping the collective can cause a low G condition (between “low positive G”, up to “negative G”), which, together with large and abrupt cyclic AFT movement, can cause the rotor plane to come dangerously close to the tail boom.
1.12.5 Damage history
“The mishap aircraft had been involved in an incident [3 years before this accident]. [Another] pilot at the time lost yaw control close to the ground, and the main rotor cut through the composite structure of the tail boom in almost its entire width.
1.12.6 Other MD600N accidents linked to tail boom strike
“Between 1996 and 2004, thirteen accidents of the MD600N involving tail boom strikes were reported. 11 of those happened on the ground, of in the course of a hard landing. One tail boom strike happened in flight during the certification test flying program, and another in-flight tail boom strike occurred while flying outside the flight envelope.
So what makes the MD600 so different from the MD500? Neither the 500D nor the 500E Rotorcraft Flight Manual includes the above quote rotor blowback / tail boom strike warnings found both in the MD600 and in the Robinson manuals. (I do not have a 520N flight manual to check there.)
Despite the optical similarity, the MD600 is considerably heavier (40% higher empty weight compared to 500E). The rotor clearance between the rotor disc and the tail boom is maybe 10 cm less. I should think, equally importantly, both cabin and tail boom are much longer, resulting in the FWD and AFT masses of the airframe to be further away from the centre than in the MD500.
This in my books would result in a situation where the (already higher) mass of the MD600 acts upon a longer lever arm, generating larger inertia resisting the fuselage to follow the FWD/AFT movement of the rotor disk.