An April 11 MV-22B Osprey crash in Morocco occurred because the pilot committed a fundamental flying error which investigators have found was rendered irreversible by a tailwind neither he nor a second pilot in the cockpit noticed, Breaking Defense has confirmed.

The Defense Department announced June 29 that the Marine Corps had ruled out any “mechanical or material failure” in the accident, in which two Marines were killed and the two pilots were injured.

“This wasn’t a tiltrotor accident; it was bad flying,” said a government source with detailed knowledge of the findings, which are still being reviewed by Marine Corps leaders.

Two military officers familiar with the findings separately confirmed that the pilot of the mishap aircraft started the sequence of events that culminated in the crash by violating an explicit instruction in the Osprey’s flight manual.

“No-kiddin’ human error is involved here,” one military officer said. Another said of the pilots: “Unfortunately they put the aircraft in a position beyond the (flight manual) limits that are advertised and that are trained to, and they made an error.”

The Osprey, which can carry as many as 24 troops loaded for combat, is called a “tiltrotor” because it points two large rotors housed in wingtip pods called “nacelles” upward to take off and land like a helicopter and swivels them forward to fly like an airplane. The ability to tilt its rotors gives the Osprey far more speed and range than conventional helicopters without a conventional fixed-wing airplane’s need of runways to take off and land.

The MV-22B that crashed in Morocco was attached to Marine Medium Tiltrotor Squadron 261 (VMM-261), based at Marine Corps Air Station New River, N.C. VMM-261 was participating in a joint military exercise with Moroccan armed forces when the accident occurred.

Just prior to the accident, with the less experienced but fully trained copilot at the controls, the Osprey had set down helicopter-style to drop off at least the second load of troops its crew had delivered that day to the same austere landing zone. As the pilots took off to return to a temporary on-shore base, the following events unfolded in quick succession:

• Under a clear, daylight sky and with no dust interfering with the crew’s view, the pilot at the controls lifted the Osprey into a hover 20 or 30 feet above the ground with the plane’s nose pointing into the wind, as it had been when the aircraft landed a few minutes earlier.
• The pilot flying then used his foot pedals to turn the Osprey in a half-circle to the right, rotating in mid-air to head in the direction from which they’d arrived. As the aircraft turned, it climbed to about 50 feet.
• As the Osprey turned, the pilot pitched the aircraft’s nose down about 10 degrees by pushing the control stick forward with his right hand. At the same time, using his left hand, he turned a small thumbwheel on the Osprey’s throttle, or Thrust Control Lever, to tilt the nacelles and rotors down from 90 degrees and brought them to an angle significantly less than 75 degrees – a position that violated flight manual limits on nacelle angles at low forward airspeed. The effect was to shift the Osprey’s center of gravity too far forward, causing the nose to plunge downward.

As the nose went down, the pilot was unable to hold it where he wanted by pulling back on the control stick because the horizontal stabilizer at the aircraft’s tail was being pushed up and forward by a 20-knot tailwind. The tailwind’s speed and direction were depicted on a digital map inside the cockpit, but vegetation in the area was too sparse to alert the pilots to the wind as they looked outside the aircraft during takeoff. The tailwind pressure on the horizontal stabilizer reduced the stick’s “aft control authority” while adding downward leverage on the nose. “By the time the pilot realized he was out of back stick authority, it was too late,” one source observed.

The aircraft plunged nearly straight down into the ground, hitting nose first. The cockpit was crushed, but the two pilots – whose identities haven’t been released – were strapped into their seats and survived, though with severe injuries. The two enlisted crew chiefs, Cpl. Robby A. Reyes, 25, of Los Angeles and Cpl. Derek A. Kerns, 21, of Fort Dix, N.J., were killed. MV-22B crew chiefs, like those on Marine Corps helicopters, ride in the back cabin and often stand during flights, secured only by a long strap attached to the aircraft to keep them from falling out if the back ramp is open.

Despite being loaded with more than half the fuel an MV-22B can carry, the Osprey didn’t catch fire after crashing, and its Crash Survivable Memory Unit, or “black box,” was recovered. The CSMU, which records aircraft operating data, showed conclusively that there were no mechanical problems with the aircraft.

“Flight data information indicates that the aircraft performed as expected and described in the MV-22 Naval Air Training and Operating Procedures Standardization (NATOPS) Flight Manual,” the Defense Department’s June 29 news release said. “The U.S. Marine Corps has determined the aircraft did not suffer from a mechanical or material failure and there were no problems with the safety of the aircraft.”

Additional analysis determined that the combination of the pilot violating the NATOPS limit on forward nacelle angle at low airspeed, the resulting excessive forward center of gravity and severe downward pitch of the aircraft’s nose, and the tailwind pushing the bottom of the horizontal stabilizer as the tail of the aircraft angled upward all contributed to the crash.

“The combination of those factors made the aircraft unrecoverable at low altitude,” one source said. “Below 40 knots calibrated air speed, there is insufficient wind moving across the conventional airplane control surfaces to add roll, pitch or yaw control authority. The aircraft relies on the controls of a helicopter.”

A helicopter’s controls change the craft’s position by changing the angle at which its rotor blades hit the air they’re moving into, known as the “relative wind.” An airplane’s controls change the angles at which control surfaces such as elevators and ailerons hit the relative wind. The helicopter-airplane hybrid Osprey’s computerized flight controls work like a helicopter’s at slow speeds and like an airplane’s once a V-22 gains enough forward air speed for its wing to produce more lift than its rotors — usually 110 knots or more with the wing horizontal. At nacelle angles between 75 and 30 degrees, the Osprey’s flight control computer blends helicopter and airplane controls.

Despite those characteristics, veteran Osprey pilots said the root aerodynamic cause of the Morocco accident wasn’t peculiar to a tiltrotor.

“As with any aircraft, there are center-of-gravity limitations,” said a pilot who agreed to discuss the crash on condition of anonymity because the results of the investigation have yet to be released. “Too much forward center of gravity in any aircraft, the aircraft will not have enough pitch authority to counter the nose-down pitching moment. Too much aft center of gravity and the aircraft will not have enough pitch authority to counter a nose-up pitching moment.”

Contrary to at least one report, the crash in Morocco had nothing to do with a well-known Osprey peculiarity known as Pitch Up With Side Slip. Aerodynamically, what happened in Morocco was the direct opposite.

In Pitch Up With Side Slip, the downwash from the rotors of an Osprey flying at a slight angle into relative wind of 20 to 30 knots, or hovering with a front quartering wind of 20 to 30 knots, can push down on the horizontal stabilizer at the tail, pitching the aircraft’s nose up. Osprey pilots are trained to compensate for Pitch Up With Side Slip by pushing the control stick forward a bit or turning the aircraft’s nose directly into the wind. The Osprey’s computerized flight controls also automatically compensate for Pitch Up With Side Slip by tilting the nacelles forward a touch when necessary.

The pilots in the Morocco crash could face penalties, depending on the findings of a Judge Advocate General Manual (JAGMAN) investigation, whose purpose is to assess responsibility, and a separate Field Flight Performance Board review, whose purpose is to determine whether the flight crew are fit, motivated and worthy of continuing to fly. The flight performance review isn’t complete. Marine leaders are reviewing the findings of the JAGMAN investigation as well as the report of an Aircraft Mishap Board, whose role is to assess safety lessons or issues raised by an accident and whose evidence is privileged and never released.

The Flight Characteristics chapter of the MV-22B NATOPS manual contains several sections relevant to the crash in Morocco, including one “Warning” — a designation used to highlight risks that can lead to injury or death.

The Warning cautions pilots that “severe pitch down and altitude loss can occur if nacelles are rotated too far forward too quickly at takeoff.”

A separate note advises that when accelerating by tilting the nacelles forward, an Osprey pilot should apply “aft stick movement to maintain pitch attitude due to thrust and cg (center of gravity) effects.”

The Operating Limitations section of the NATOPS manual explicitly instructs: “When transitioning to forward flight from a hover, limit nacelles to greater than 75 degrees until 40 KCAS (Knots Calibrated Air Speed) is reached.”

The Defense Department issued its unusual June 29 statement about the Morocco crash because the Marine Corps has long planned to deploy the first of two 12-aircraft squadrons of Ospreys to Okinawa this summer. The MV-22Bs are to replace CH-46E Sea Knight and CH-53D Sea Stallion helicopters based at Marine Corps Air Station Futenma.

Local officials on Okinawa, where a CH-53D crashed in 2004, injuring three Marine crew members but harming no civilians, have expressed concern about the Osprey’s safety record in the wake of the Marine Corps crash in Morocco and the June 13 crash of an Air Force CV-22B at Eglin Air Force Base, Fla. The CV-22B accident, which occurred during a training flight, injured the crew of five and destroyed the aircraft but killed no one.

Breaking Defense has previously reported that in the CV-22B crash, still under investigation, one possibility being examined is that the pilots flew closer to an Osprey in front of them than the prescibed separation of 250 feet. Under such circumstances, turbulence created by the rotor downwash of the lead Osprey can knock the lift out from under one rotor of a trailing V-22, causing the aircraft to roll suddenly in that direction. At low altitude, such roll offs can be unrecoverable.

After being briefed on the findings of the investigation into the Morocco crash, Japanese officials agreed to allow an initial squadron of Ospreys to be shipped to Marine Corps Air Station Iwakuni on the Japanese mainland for eventual deployment to Okinawa. In exchange, the United States pledged that no V-22s would fly over Japan until the findings of investigations into the Morocco and Florida crashes are made public.

The June 29 DOD statement also emphasized that Marine and Air Force Ospreys would continue flying elsewhere in the world, and four MV-22Bs are at this week’s Farnborough International Airshow southwest of London to provide demonstration rides to potential foreign buyers. The Naval Air Systems Command and the Osprey’s makers, 50-50 partners Bell Helicopter Textron Inc. and Boeing Co., have high hopes of making an initial foreign sale of V-22s soon. Foreign orders would lower the Osprey’s current unit cost from about $67 million for an MV-22B and $78 million for a CV-22B.

Critics who distrust tiltrotor technology or would prefer to see the money the Marines and Air Force spend on Ospreys go to other uses have long contended the V-22 is unsafe. Based on major mishaps per flight hour, however, the MV-22B has been one of the safest rotorcraft the Marine Corps flies over the past 12 years, with the crash in Morocco the service’s only loss of an Osprey since Dec. 10, 2000.

Even counting two crashes of Air Force CV-22Bs in the past two years, the Osprey’s safety record has been exceptionally good since the aircraft was redesigned and retested a decade ago. Since Oct. 1, 2001, three Ospreys have crashed with a loss of six lives. During the same period, the U.S. military has lost 414 helicopters at a cost of 606 deaths.