The information being released by the Marine Corps continues to give increasing indications that 'asymmetrical settling with power' was the cause of the April V-22 accident in Arizona. As pointed out above, when the vortex ring state develops on one side, the result would be a dropping of the wing on that side. The instinctive reaction of the pilot would be to use lateral cyclic stick in the opposite direction of the falling wing. This causes an increase of collective pitch on the rotor system that is in the vortex ring state. As we know, that is exactly the worst thing to do when a settling with power condition exists as it further worsens the resultant reduction in lift provided.
The existence of this phenomenon is certainly not a death blow for tiltrotor technology. It is a limitation that contributes to the defining of the normal operations flight envelope. Compare the above situation to what happens to an airplane when a poorly trained or inexperienced pilot allows the aircraft to stall while on approach for landing. If the airspeed is allowed to reduce to the point of aerodynamic stall (critical angle of attack is reached) the airplane will begin to lose altitude, and it is very likely a wing will drop. If the pilot then does what is instinctive -- pulls back on the yoke to stop the loss of altitude, and applies opposite aileron -- the aircraft will enter a stall/spin and at that point, lacking the altitude necessary for recovery, the pilot will be powerless to prevent the aircraft from impacting the ground. Unfortunately low time pilots repeat this scenario many times every year resulting in numerous unnecessary deaths. This does not prove that all airplanes are dangerous, or indicate that they should all be permanently grounded.
In the same way that a fixed-wing a/c should not go below 1.3 times the stall speed when on final approach, a single rotor helicopter should not descend at a rate of 300 fpm or more at slow speeds. Violating these limitations at low altitude has time and again resulted in the same devastating consequence experienced by the V-22 in Arizona when it was operated outside of its normal flight envelope. Does this mean that the use of all helicopters should be stopped? The characteristic of 'asymmetrical settling with power' in tiltrotors is by no means a fatal flaw necessitating the cancellation of an aircraft that more than doubles the range and speed capabilities of the helicopters they are replacing. When you develop an aircraft with dramatically increased capabilities at the high speed end, it is quite typical that there will be low speed limitation penalties that accompany the advances. However, it goes without saying that accidents with loss of life are not an acceptable price to pay for an increased capability to deploy troops further and faster than ever before.
The undeniable answer to preventing accidents in the future is training and experience. Pilots must learn to recognize the conditions that will put them outside the normal flight envelope, and apply correct recovery techniques when appropriate. All aircraft flying today have 'dangerous' characteristics in certain flight regimes that must be avoided, regardless of whether you are in an airplane, helicopter, or tiltrotor. When pilots are not properly trained to understand and recognize the conditions to be avoided, disastrous consequences are inevitable.
The Marine crew that perished in the April crash had a number of contributing factors working against them. The most obvious is a lack of experience in tiltrotors, having only 80 or 90 hours each in the V-22. Combine that with the extremely high workload environment of flying at night, flying with night-vision goggles, and flying in formation in a simulation of a high pressure rescue/extraction of civilian hostages in the final phase of an operations evaluation. Operating under these circumstances would be difficult for highly experienced tiltrotor pilots. Like many military operations, this type of flying will always be categorized as high risk, particularly when you throw in a real life high-threat location. But in time missions such as this will be able to be carried out successfully by those pilots who are now gaining experience in the military. It is terrible that 19 people lost their lives in an accident that should not have happened, but hopefully the lessons learned will save lives in the future.
Before any judgment is made on exactly what happened, why it happened, and how do we prevent it from happening again, we need to wait for ALL of the facts to be in. Fortunately there was a FDR, CVR, an eye witness on the ground, and the entire event was being filmed in infrared by an F/A-18 flying overhead at altitude. So it is likely that all the facts needed for complete analysis will be available, and that appropriate evaluation and recommendations will be forthcoming. The Marines more than anyone are interested in ensuring that this event never occurs again. The V-22's return to service shows their confidence in tiltrotor technology. That confidence comes from 45 years of tiltrotor research, including 11 years of flying the V-22.
R. Denehy, ATP - ASMEL, ASES, RH; CFI - ASM, RH; CFII - A&H