CV-22 accident analysis 12-20-10

Golden Rules Known and Not Known
• Every fixed-wing pilot knows that it takes more power to
go slower (safely) when flying “behind the power
curve”, i.e., slower than some critical speed.
• What every rotorcraft pilot does NOT know is a similar
rule for rotorcraft that states it takes more power to
descend safely faster when flying “ahead the thrust
curve”, i.e., flying with a rotor disc angle of attack
greater than some critical angle (safely is the key word
here). (see slide 6)
What Happened to A/C 06-0031 and Why
Due to being late, high, and fast, the pilot of A/C 06-0031 elected to execute a tactical approach
with a quick stop at the bottom. This is a common maneuver to avoid hostile fire (actual or
anticipated) (Remember, this was the, or one of the, first combat mission)
The quick-stop maneuver, if performed poorly will end up with too high airspeed and too high rate
of descent at the critical point just before landing
In this situation, the rotor disc angle of attack can be very high
The high rotor disc angle of attack translates to rotor conditions mimicking a much slower forward
speed and much higher vertical speed. This places the helicopter rotor(s) into, or much closer to,
the VRS domain envelope. (see slide 5)
Prior to landing, the crew would have computed the maximum power available and the power
required for the landing conditions. However, at the conditions resulting from a poorly executed
quick-stop, maximum available rotor thrust (as distinct from power) is significantly reduced (by as
much as 30%) and the aircraft response to collective pitch is significantly reduced.
In a conventional helicopter, at this point the pilot will typically “droop” the rotor(s) (i.e., decrease
the rotor RPM due to too asking for more power than the engines can deliver). This allows
energy stored in the rotor to cushion the landing. Typically, a hard, but safe landing results.
In V-22 all the above is the same, but the rotor cannot droop as the flight control computer will
remove collective pitch in order to maintain rotor RPM. This will result in a very hard, and usually
fatal touchdown.
Figure 1:
Tactical Approach with Quick-Stop Maneuver
Relative Wind
Critical phase of maneuver:
At this point, rotor disc angle of attack is high
with airspeed still high. In this flight condition,
rotor trust is decreased by back-flow through the
rotor. Typical pilot error is to end up at this point
with too much airspeed and too much rate of
descent. In most helicopters, the pilot will droop
the rotor speed in attempting to make a safe
landing. In V-22, this is much more dangerous as
the rotors have no energy stored, and the flight
control computer will remove engine power in
order to maintain rotor speed.
Figure 2:
A/C 06-0031 Impact Point Rotor-Flow Geometry
Rotor Saw This
VA = 3800 FPM
58 kts
VT = 47 kts
1800 FPM
Pilot (and accident board) Saw This
Figure 3:
Test Data and A/C 06-0031 Position on VRS Plane
Accident investigation board states
the aircraft was here at impact.
Actually, the aircraft
was here at impact
Available Thrust Reduction Due to Rotor Backflow
Rotor Thrust vs. Rotor Angle of Attack at Fixed Power – NASA data
Rotor Thrust
Collective Pitch
Rotor Disc Angle of Attack
Notice rotor thrust decreases rapidly past 30 rotor disk AoA.
Some Comments for the Record
As stated in the Mishap Report, only a dual engine failure in the V-22 can result in low
RPM of the prop-rotors.
The speculation that the mishap aircraft had a dual engine failure on short final is
nothing short of idiotic.
Destroying the aircraft, and the data recorder, to protect the technology was stupidity
of the highest order. What technology were we protecting? From whom? What Taliban
scientists were ready to exploit this technology. Stupid is the only word applicable!
The estimates of rotor PRM and ground speed are suspect. This analysis was not
released. I suspect there are problems with it and rotor RPM was normal and ground
speed significantly lower. This analysis needs to be redone by independent,
competent authority.
The level of ignorance on the subject of VRS, in both the USAF and USMC, is
Pilots have no clue to what is going on, aerodynamically, in non-equilibrium situations
such as the quick-stop maneuver. And, they are precluded from experiencing them by
restrictive training rules.
Everything stated in these slides is elementary aerodynamics for rotorcraft and should
be common knowledge for all rotorcraft pilots – it is not.
Until this becomes common knowledge, this accident will recur many times in a
combat environment, as I have pointed out repeatedly for the past 10 years (to deaf
ears). The alternative is to take the USMC road and use the V-22 as a “Combat
Circulator” for which it is well suited.

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