Digital photo titled unloading-2

Helicopter Accident Investigation

by Philip Greenspun, Ph.D., ATP-H, CFII-H and Adam Harris, A&P and IA; updated September 2012

Site Home : Flying : One Article

This article describes a methodology for investigating helicopter accidents, with particular emphasis on the popular Robinson R22 and R44 helicopters and some examples drawn from the Bell 206 JetRanger.

Establish the Maintenance Status of the Helicopter

Maintenance and mechanical-related accidents account for only a small percentage of all helicopter accidents, perhaps as low as 10 percent in the case of Robinson helicopters. Nonetheless, it is a straightforward matter to start by investigating whether or not the helicopter was legally maintained by the operator.

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.

Establish the Currency of the Pilot

Pilot proficiency degrades quickly if not used. Via FAR 61, the FAA sensibly requires the following before a person can act as pilot in command of an aircraft carrying passengers: Insurance companies often impose more stringent requirements, e.g., recurrent training on an annual basis. In order to guarantee the ticket-buying public a higher level of safety, the FAA similarly imposes more stringent requirements on pilots of commercially-operated aircraft, e.g., ground and flight tests every 12 months for charter pilots (FAR 135.293) and proficiency checks every 6 months for airline pilots (FAR 121.441).

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.

Establish what happened

In my opinion, the first place to look for information regarding what happened is within the witness statements and pilot report (Form 6120) collected by the NTSB shortly after the accident. These are gathered within 10 days after the accident and therefore may be more reliable than, for example, deposition testimony collected a year or two later.

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.

Loading and unloading

A depressing number of helicopter accidents occur before the helicopter has left the ground. It is difficult to restart a hot jet engine and therefore many times people are loaded or unloaded while the blades are spinning. This can lead to fatal encounters with main and tail rotors. This safety briefing that we hand out to sightseeing customers at East Coast Aero Club explains some of the hazards and how to avoid them. When investigating an accident, establish whether or not passengers were "hot loaded" and, if so, what kind of a safety briefing did they receive prior to being led out to the vicinity of the helicopter.

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.

Landing Zone Selection and Preparation Problems

One reason why helicopter accident investigation can be more challenging than airplane accident investigation is that helicopters can be landed away from the structured environment of an airport. An airport is a vast area cleared of obstacles. The approaches to the runways are kept clear of trees, buildings, and other obstacles by FAA regulation. There are no such guarantees when landing in someone's backyard. In addition, airports are generally carefully cleared of debris that could be disturbed by rotor wash and sucked into engines or blades. It does not take a heavy object to damage a helicopter. One operator here in New England threw an empty water bottle out of his Robinson R22. It was sucked up into the rotor system and made a large enough dent in a blade to require a $10,000 replacement. (I imagine he has a new attitude toward littering.)

"Preparing a Helicopter Landing Zone" explains some of the considerations for pilots and folks on the ground.

Bad weather

Nearly all small helicopters are certificated for VFR (visual flight rules) operation and cannot be legally flown through clouds. The pilots of small helicopters generally do not hold instrument ratings or, if they do, they are not current and proficient at flying by reference to instruments. Unlike airplanes, helicopters are unstable. When a Robinson or JetRanger helicopter enters the clouds, therefore, the aircraft is probably low to the ground, lacking the basic instruments for instrument flight rules (IFR), piloted by someone who has little or no experience flying in clouds, and tending to pitch or roll.

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!)

Hitting Obstacles

Hitting powerlines or similar obstacles is a common cause of helicopter accidents. The FAA does not have specific altitude limits for privately operated helicopters. For air taxis, FAR 135.203 limits helicopters to at least 300' above a "congested area". At the Robinson Factory Safety course, the company recommends at least 500' above the ground unless a pilot is very familiar with the area. This is partly because any obstacle more than 500' above the ground must be marked with high-power anti-collision lights.

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?"

Mitigating the Effects of a Mechanical Failure: The Height-Velocity Curve

Following a power failure, e.g., the engine stops when the gas tank runs dry, a properly trained pilot should be able to enter either a hover autorotation or a standard autorotation, depending on altitude and airspeed. Depending on the design of the helicopter, notably the weight and therefore amount of energy stored as blade inertia, it may not be possible to land the helicopter without damage after autorotating. FAR 27.87 states "If there is any combination of height and forward speed (including hover) under which a safe landing cannot be made [after a power failure], a limiting height-speed envelope must be established...". This "height-velocity diagram" or "dead man's curve" is included in the Pilot's Operating Handbook (POH) or Approved Flight Manual (AFM) and the pilot is instructed to avoid operating at potentially dangerous combinations of airspeed and altitude.

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, 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."

Accidents while Hovering

Hovering is a hazardous phase of flight due to three factors: (1) low airspeed, (2) proximity to obstacles, and (3) high skill level required depending on wind conditions.

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.

Accidents while Landing or Taking Photos

Helicopters that are simultaneously moving slowly and descending are subject to a hazardous condition known as "settling with power" or "vortex ring state". The helicopter sinks into its own previously disturbed air and the result is a dramatic increase in descent rate that cannot be arrested by adding collective pitch and engine power.

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.

Special Hazards of Two-bladed Helicopters: Mast Bumping

Two-bladed helicopters, e.g., the Huey, Bell JetRanger, and Robinsons, are subject to a hazard known as mast bumping. Due to gravity, the helicopter hangs from a "teetering" rotor system. The pilot flies the rotor system and the helicopter fuselage follows, giving the pilot the illusion that he is flying the helicopter. If gravity is no longer acting on the helicopter, i.e., if the helicopter is in a "low G" condition, the tail rotor will tend to roll the helicopter to the right. The alarmed pilot will reflexively apply left cyclic, which causes the rotor system to tilt, but, without gravity, the helicopter fuselage won't follow. This puts the rotor blades and the mast into an extreme orientation and an inner portion of the blades will smack into ("bump") the stationary mast that is holding up the rotor system. The best case result of this encounter is expensive damage; the worst case is separation of the rotor from the helicopter and death of everyone on board.

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.

Special Hazards of the Robinson R44: Governor runs off Engine Tachometer

The Robinson R44 has a potentially lethal design defect: the electronic governor, which is supposed to ensure that rotor speed remains at a safe "100%", is not hooked up to the rotor tachometer. It is hooked up to the engine tachometer. To make matters worse, the engine tachometer is driven by a set of mechanical points inside one of the ignition magnetos. This was state-of-the-art for the Model A Ford, but by the mid-1980s piston-powered aircraft were generally running the tachometer from a much more reliable optical sensor. Despite the critical nature of this system, Robinson never adopted a more reliable non-mechanical way of measuring engine speed nor incorporated rotor speed measurement into the governor's circuitry.

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.]

Special Hazards of the Robinson R22: low-inertia rotor system

Designed as a rich guy's transportation helicopter, the Robinson R22 quickly became instead the world's most popular training helicopter. It also happens to be one of the most dangerous machines in which to train. Why? Much of helicopter training is practicing autorotations, in which the engine is twisted toward idle and the helicopter glides toward the ground. This maneuver requires the student to adjust the collective pitch control to maintain rotor RPM within safe limits. The heavier the rotor blades the more stable their speed will be due to inertia. With a low-inertia system, the student will have to react very quickly to a simulated engine failure or the simulated emergency becomes a real emergency. In the R22, the student and instructor have 1.6 seconds to act after a simulated engine failure. If neither lowers the collective and enters an autorotation within that 1.6 seconds, both are likely to die.

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?"

Suggested Deposition Topics

When deposing a pilot witness, here are some topics that the pilot should be familiar with from his or her training: If there is any question of the accident involving aerodynamics or pilot actions, I strong recommend that the attorney taking the deposition avail himself of at least two or three helicopter lessons, with associated ground school reading (mostly the FAA Rotorcraft Flying Handbook). This will cost roughly $1000 and significantly reduce the possibility of confusion during a deposition where the attorney may say "collective" and mean "cyclic", for example. Everything about helicopters makes a lot more sense once you've had your eyes on the machine and your hands on the controls. (See my list of recommended flight schools; one might be near your office!)


Investigating a helicopter accident should start with a thorough examination of the helicopter's logbooks by an IA and the pilot's logbooks by a helicopter CFI (certificated flight instructor). Was the machine legal to fly on the day of the accident? Was the pilot legal to fly that machine? Once those questions are answered, proceed to the NTSB docket (list of FOIA dockets) to see what the pilot and witnesses said about what happened. Then look at the NTSB factual report. After that, each investigation is going to differ, depending on the circumstances and machine involved.

Keep an open mind until the day that you complete your expert report (and even after, if new evidence is presented).

About the Authors

Philip Greenspun holds FAA Airline Transport Pilot and Flight Instructor certificates for both airplanes and helicopters. He has served as Chief Instructor for East Coast Aero Club's helicopter school, as pilot for a helicopter charter operation, and as an airline pilot for a Delta Airlines subsidiary. Greenspun is type-rated in the Canadair Regional Jet and Cessna Citation Mustang and has nearly 4000 hours of flying experience (more). Greenspun has a Ph.D. in Electrical Engineering and Computer Science from the Massachusetts Institute of Technology.

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.

Text and photos Copyright 2003-2012 Philip Greenspun and Adam Harris.