Rotor Blade tree pruning and the Height-Velocity Diagram for the MD500 Helicopter

Two recent helicopter accidents, involving commercial contractors, highlight the need for improved safety standards. In an accident that happened in Ohio, on July 29th, a helicopter crashed while doing aerial tree pruning, needlessly injuring a helicopter pilot. In the other helicopter accident, near Wenatchee, WA on July 23rd, a pilot was killed when he crashed while flying low over trees to air-dry a cherry crop. Both accidents would not have happened if FAA and NTSB were properly regulating the helicopter industry.

20140729.. pic demonstrating rough limb pruning by Rotor BladeThis Ohio accident shows a system used by a company (Rotor Blade) to trim off tree limbs to form tall facewalls, such as along powerlines. As shown in the photo of a pine tree at right, the quick pruning is rough, leaving large stubs.

The cutting system is a tall stack of blades, said to spin at roughly 5,000 rotations per minute. The blades appear to be between 24-30″ in diameter.20140729.. pic demonstrating large blade system used by Rotor Blade20140729.. pic demonstrating MD500 takeoff by Rotor BladeIn the accident in Ohio on 7/29/2014, the Rotor Blade helicopter had been hired to prune the forest edge along a new recreational trail. The MD500 lost power and fell into the trees. The pilot was injured, but likely would have been killed, if not for the way the trees slowed his crash.

According to the Height-Velocity Diagram created by the manufacturer and approved by FAA, the pilot was clearly supposed to avoid operations such as this tree pruning. The red ellipse marks the approximate flight parameters for tree pruning: at roughly 100-ft altitude and with slow speeds, typically less than 5-10 knots … which is right in the most dangerous area of the cross-hatched ‘AVOID’ portion of the diagram! Note that the recommended flight profile (marked in green) indicates the helicopter should have been travelling with a speed of at least 60 knots when at the 100-ft altitude for pruning.
20140730.. Height-Velocity Diagram for MD500, with markups
A similar height/velocity diagram for the Bell 206 helicopter makes the same point: pilots are to avoid low/slow operations, such as using helicopters to dry cherries.

So, Why do Accidents Like this Continue to Happen?

Mostly because FAA and NTSB continue to ignore this inappropriate use of helicopters. The pilots fly to make money and build flight hours that help them eventually get better jobs. If they protest the safety, the operator just replaces them with another pilot. The pilots cut safety corners and just ‘hope’ that nothing bad happens while they are flying their jobs. Meanwhile, the insurance companies and the operators do just like FAA: they pretend to not notice the safety problems. If anything happens, hey, blame it on the pilot. And the public agencies who hire these dangerous contractors? They, too, look the other way, assuming that by hiring a contractor, they are not culpable for the injuries and fatalities that eventually result. It is sort of a ‘safety trickle-down’, where the margin of safety reduces to near-zero.

Is There a Better Way?

Yes. Some jobs are practically done with helicopters, and some are not. This tree pruning system looks to be a bad idea. Why pay a company to do a fast but crude job pruning trees with helicopters, when it can be done to a much higher quality while employing trained professionals and using much less energy? If FAA and NTSB would press harder to reduce helicopter fatalities, the helicopter operators would not be allowed to fly this way. And, as a big benefit, there would be more real, physical jobs for tree-service professionals who are not aviators.