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Helicopter Accident Investigationby Philip Greenspun, Ph.D., ATP-H, CFII-H and Adam Harris, A&P and IA; updated September 2012 |
Like all civil aircraft in the U.S., the helicopter must have had a annual inspection within the preceding 12 months, signed off by an FAA-certificated mechanic with Inspection Authorization (IA) or by an FAA-approved Repair Station. If the most recent "annual" was signed off January 12, 2011, for example, the machine is legal to fly through January 31, 2012. Similarly, the helicopter's transponder must have been tested in accordance with the FARs (e.g., 91.413) within the preceding 24 months.
There are additional maintenance requirements for commercially-operated helicopters, including those at flight schools. They must have inspections every 100 hours or, in some rare cases, on a different schedule if the flight school has received FAA approval to conduct progressive inspections. Finally there are manufacturer-recommended maintenance procedures, typically at 25-, 50- and 100-hour intervals. In most cases, these are not legally required for operators, but may serve as a reference for a FAR 135 or FAR 121 maintenance manual.
Federal Aviation Regulations (14 CFR but generally "FARs") restrict maintenance on certified aircraft to FAA-certificated mechanics and repair stations. The only exception is that certificated pilots may, under FAR 43.3, perform some preventive maintenance where "preventive" is defined with an explicit list of tasks in Appendix A to FAR 43. If the pilot were to perform, for example, an oil change, he or she is required to log the maintenance in the aircraft's logbooks and include his or her certificate number in the log entry.
Did you notice any equipment in the helicopter that was not factory-standard? If so, you should be able to find paperwork for a Supplemental Type Certificate (STC) or FAA Form 337.
Bad or inconsistent paperwork doesn't cause accidents, but aircraft that have received high quality maintenance almost always have high quality paperwork.
Who can perform this analysis? The best qualified person to conduct a logbook and aircraft wreckage inspection is an FAA-certified IA with experience in buying and selling used aircraft. The kinds of logbook anomalies that might explain an accident are similar to the kinds of logbook anomalies that experienced mechanics look for during a "pre-buy" inspection of an aircraft.
For pilots of Robinson helicopters, the FAA imposes an additional range of training and recurrent training requirements via "SFAR 73 to Part 61", which was written into the law on February 23, 1995 after a rash of accidents involving Robinson helicopters and three special certification reviews by the FAA (source: National Transportation Safety Board Special Investigation Report PB96-917003 NTSB/SIR-96/03, "Robinson Helicopter Company R22 Loss of Main Rotor Control Accidents"). Under SFAR 73, a Robinson pilot must be signed off by an instructor act as pilot in command of a particular model of Robinson helicopter. Pilots with less experience must have a flight review every year rather than the standard two years. Any flight review must be accomplished in the precise model of Robinson helicopter to be flown. So, for example, a pilot who flew a Robinson R22 for 500 hours then upgraded to a four-seat R44 and flew it for a few years would not be legal to get back in and fly the R22 unless he or she first did a flight review with an instructor. An instructor cannot teach in the Robinson R22 or R44 unless he or she has been specifically signed off for that privilege by an FAA-designated examiner.
Regardless of legal or insurance requirements, a generally prudent helicoper pilot's logbook will show a reasonable amount of recurrent training. What's reasonable? A guy who flies his family to a vacation house every weekend will probably need at least a few hours annually to brush up on emergency procedures. A working flight instructor who is teaching emergency procedures regularly to students might need only a quick check.
All of the required information to establish pilot currency should be in a pilot's logbooks and FAR 61.51 requires that such logbooks be kept.
The pilot's own report can be very informative. Was the operation conducted with a prudent fuel reserve? Was the pilot familiar with the gross weight limitation of his or her aircraft? Does the pilot's estimated weight at the time of the accident make sense given the fuel, passengers, and bags on board?
The NTSB factual report is also helpful, but keep in mind that the agency does not go all-out investigating the crash of a 2-, 4-, or 5-seat helicopter. Sometimes they delegate fact-finding to local FAA personnel, to factory representatives (guess how likely they are to find that the problem was related to a manufacturing defect!), or to operators. Oftentimes the report will simply say, for example, that an engine stopped "for unknown reasons".
The NTSB probable cause report is not admissible in court and cannot be relied upon by an expert witness.
Also look at how the pilot parked the helicopter. We train our students always to park with the nose of the helicopter pointed at the building or wherever else the passengers are trying to walk. This way they have no reason to walk past or go near the tail rotor, the most dangerous part of the helicopter when on the ground.
"Preparing a Helicopter Landing Zone" explains some of the considerations for pilots and folks on the ground.
Even experienced instrument airplane pilots have trouble when inadvertently entering the clouds. They might be capable of planning and executing an instrument flight, but when forced to transition suddenly from outside visual references to the gauges inside the aircraft, they become disoriented. As noted previously, the helicopter is more difficult to control on instruments and the pilots are much less experienced with instrument flight. At our flight school, for example, we have interviewed instructors holding CFII-H ratings, i.e., the FAA thinks they are qualified not only to fly by reference to instruments but also to teach new instrument students. When asked to don a hood (like a baseball cap pulled down low) so that their view of the natural horizon is obscured, some have quickly gone into steep banks and essentially lost control of the helicopter. This despite the fact that their entry into the simulated clouds was entirely planned. (We didn't hire these folks, by the way!)
Sometimes it is necessary to fly low, e.g., for takeoff and landing or possibly for a photography project. However, if the accident helicopter was flying low simply while getting from Point A to Point B, it is worth asking "Why?"
As an example, look at this Bell 206 JetRanger height velocity diagram (taken from a U.S. Army manual for the identical OH-58). The diagram is divided into three areas: safe, caution, and unsafe. According to the manufacturer, it is not safe, for example, to be hovering (zero airspeed) 50' above the ground. Nor is it safe to be zipping along at 90 knots, 10' above the ground.
How can we apply this diagram? Let's look at the NTSB factual report regarding the crash of a Bell 206B conducting a photo flight on September 11, 2007 off the coast of Sarasota, Florida. "The boat captain stated that the helicopter was flying at about seven to ten feet off the water, about 100 yards in front of and to the left of the boat. They were traveling, at his estimate, about 85 mph". Without reading any further, we know that the pilot had set up his passengers for trouble in the event of a power failure. The height-velocity diagram said "don't be any lower than about 30' when going faster than 40 knots" and the pilot was just 7-10' above the water. The actual accident seems to have been caused by the pilot catching a skid in the water, but the "accident chain" started with his flying lower than the manufacturer considered prudent at that airspeed.
The FAA's Rotorcraft Flying Handbook, available online from http://www.faa.gov/library/manuals/aircraft/, is another good source for what constitutes prudent piloting of a helicopter, e.g., "During the (normal) takeoff, fly a profile that avoids the cross-hatched or shaded areas of the height-velocity diagram."
The low airspeed during a hover makes it problematic to land the helicopter without damage or injury in the event of a power failure. A standard autorotation takes advantage of kinetic energy stored in forward airspeed for a "flare" just before landing. With no airspeed (i.e., a hover), there is no kinetic energy for the pilot to use. The FAA Rotorcraft Flying Handbook suggests a "normal hovering altitude" of 2-5 feet. Any certified helicopter should have enough rotor inertia to permit a smooth landing after an engine failure from this altitude. Even if a pilot does nothing, common sense suggests that it is safer to fall off a 2'-high curb than off a 20'-high roof. The emergency procedures section or height-velocity diagram in a Pilot Operating Handbook may provide guidance as well. For example, the Robinson R22 and R44 POHs offer only one emergency procedure for power failure in a hover, titled "Power Failure Below 8 Feet AGL", implying that this is the highest altitude from which a safe landing has been demonstrated. Helicopters with higher inertia rotor systems, e.g., a Blackhawk or Huey, can be landed from a higher hover.
Proximity to obstacles presents a unique hazard to helicopters. If the helicopter contacts a fence post or other obstacle that prevents the helicopter from translating sideways, the slightest bit of pivot will put the rotor system in a position where the power of the helicopter engine is actually pulling the helicopter around the pivot and into the ground. Pilots are trained to lower the collective pitch control, taking the power out of the rotor system, when they sense an impending dynamic rollover, but it happens very quickly.
Hovering a helicopter is the hardest skill for a beginner pilot to learn. Holding a steady hover in left crosswind can be a challenge even for an experience pilot. A beginner usually has little trouble conducting basic maneuvers 1000' above the ground and moving forward at 70 knots. That's partly because the helicopter tail has horizontal and vertical stabilizer surfaces and partly because minor attitude variations don't matter much, e.g., the helicopter flies just as well at 60 or 80 knots as at 70 knots. In a hover, the stability provided by air flowing over the tail is gone and minor attitude variations lead to alarming translations over the ground. In a strong gusty wind, the helicopter moves in and out of Effective Translational Lift (ETL), becoming dramatically more or less efficient. This requires large collective adjustments to maintain hover height, since the amount of power required to hold a 5' hover without ETL will cause the helicopter to fly away if the wind or a pilot-induced drift causes the helicopter to get into ETL. A separate issue is that the helicopter is reasonably stable when hovering nose-into-the-wind. If, however, the pilot deems it necessary to rotate the helicopter, perhaps to fit into a conventional parking space, the helicopter can be difficult to control. The left crosswind is the worst for an American helicopter such as the Robinson or JetRanger; it blows disturbed air pushed sideways by the tail rotor back into the tail rotor. In a strong enough left crosswind, even the world's best helicopter pilot may not be able to maintain control while hovering.
Settling with power most commonly occurs on steep approaches to confined landing zones (i.e., not at an airport) or during photography flights where a pilot believes himself to be hovering but is in fact sinking.
Pilots receive some ground school education on how to recover from a low-G condition, but the prudent way to avoid mast bumping is by avoiding the situations that will tend to cause a low-G condition. Extreme maneuvers by a pilot can result in the helicopter going low-G, but a more common source of reduced G forces in two-bladed helicopters is turbulence. Did the accident flight occur on a day when the wind was gusting to 30 knots and the FAA had issued an AIRMET for turbulence? If so, look into the possibility of mast bumping.
What's the practical consequence? If the points within the magneto go bad, the engine tachometer will give unreliable readings, causing the governor to open and close the throttle more or less at random. A quick-minded pilot can disable the governor, of course, but the situation can develop very quickly and there is nothing in the Robinson training that would prepare a pilot for this extremely hazardous condition. If the rotor speed decays below about 80%, which can take just a few seconds without engine power, the helicopter is certain to crash and no pilot action can recover it.
[The authors believe that this design defect exists in most models of the R22 as well, though early R22s may have an optical sensor in the transmission.]
The engineer behind the R22, Frank Robinson, stated repeatedly that he did not design the R22 as a trainer and did not want people using it as a trainer, suggesting that they use the more expensive R44 instead (it has about 4 seconds of rotor inertia instead of 1.6 seconds). However, flight schools using the R44 could generally not compete with those using the R22 (a brand-new student can tell the difference between $200/hour and $400/hour, but he or she probably can't understand the practical consequences of lower rotor inertia). Thus the R22 continues to be used at flight schools around the world and its low-inertia rotor system and lack of power reserve result in frequent accidents during practice autorotations.
Despite having had more than 30 years of experience with flight school interest in the R22, Robinson has never engineered a version of the machine with a high-inertia rotor system. Despite selling all of its helicopters with restrictive contracts governing their use, Robinson does not attempt to restrict flight schools from purchasing R22s and using them for teaching simulated engine failures and practice autorotations. Robinson's attempts to limit the number of accidents have been paperwork-based, e.g., adding cautionary safety notices to the back of the POH or working with the FAA to establish additional training requirements for pilots and instructors in the R22.
When investigating an R22 training accident, ask "Would this have happened with a higher inertia rotor system? With a larger power reserve?"
Keep an open mind until the day that you complete your expert report (and even after, if new evidence is presented).
Adam Harris holds Airframe and Powerplant as well as Inspection Authorization FAA certificates for aircraft maintenance. He also holds Commercial Pilot and Flight Instructor certficates. Harris started flying in 1990 and has been Head of Maintenance at East Coast Aero Club since 1999.
The authors have served as expert witnesses in litigation involving helicopter accidents, e.g., in Bouret et al. v. Robinson Helicopters and Caribbean Aviation Maintenance (United States District Court, District of Puerto Rico), which culminated in a 6-week jury trial in March 2012. The authors conducted their investigation on behalf of defendant Caribbean Aviation Maintenance, which the jury found not liable.