Tailboom chop in the New York City tour helicopter crash

by philg

Friends have been asking for an explanation of the horrific recent NYC tour helicopter crash, in which the machine was seen falling without its empennage (i.e., the tail had been chopped off by the main rotor system). Here are the passengers just before departure:

The weather seems to have been pretty good. The crash was at 3:15 pm, 15:15 Eastern or 19:15Z. Here’s the nearby (LaGuardia and Newark) weather at 1851Z:

KLGA 101851Z 16017G22KT 10SM FEW040 BKN100 BKN140 OVC250 08/M04
KEWR 111851Z 04011KT 7SM -RA BKN010 BKN017 OVC032 08/05

LGA had winds gusting 22 knots, which can generate some turbulence near buildings. Newark had steady winds of 11 knots with light rain and a low ceiling of 1000′ (not a problem for a helicopter that will operate at 500′). If turbulence leads to a low-G condition and the pilot doesn’t react properly, the result can be mast bumping (inner portion of the blades hit the shaft holding up the rotor system), but that common initial speculation seems unlikely in this particular case because the winds weren’t that heavy and a tourist flight isn’t usually the time for intentional rollercoaster-style maneuvers.

“Pilot in Hudson River helicopter crash called about needing fuel before fatal accident, CEO says” (Fox):

“The pilot of the doomed aircraft reportedly radioed about needing to refuel minutes before the helicopter crashed into the chilly waters, according to New York Helicopter Tour CEO Michael Roth, whose company operated the helicopter.”

This points to a more likely scenario (albeit still complete speculation until further data are available): engine stoppage due to running out of fuel followed by a failure to initiate an autorotation. The Robinson R44 has a similar two-blade rotor system to the Bell 206L4 (N216MH) that crashed. Here are some excerpts from the pilot’s operating handbook for the R44 that lay out the accident sequence:

  1. fuel exhaustion causes the engine to quit
  2. the engine quitting causes the rotor system to slow down
  3. the pilot, startled by the engine quitting, does not immediately (within 2-4 seconds) enter an autorotation, a lowering of the collective pitch control that flattens the blades and allows them to maintain speed while windmilling
  4. the retreating blade stalls, partly due to the high angle of attack caused by the relative wind beginning to come from below the helicopter as the helicopter falls, and the rotor system “blows back” due to the lost of lift (explaining this in full requires some understanding of the physics of gyroscopic precession)
  5. the severely tilted rotor contacts and chops off the tailboom

If this is indeed what happened, what can we learn? First, it is a lot easier to do the right thing in a training environment when simulated emergencies (e.g., an instructor rolling the throttle to idle) are expected. Second, being 100 percent vigilant 100 percent of the time is a perfect job for a computer whose job can be to shove the collective down and enter the autorotation even if the human pilot is still startled and frozen. A system like this has been designed, but is not widely available. See “HeliTrak launches R22/R44 Collective Pull Down” (2018), for example.

You might ask how it is possible for an experienced pilot to run an aircraft out of fuel. I was providing some recurrent training to two Brazilian helicopter pilots. Brazil is a huge market for helicopter taxi service due to horrific traffic and high levels of crime (they could use an El Salvador-style clean-up!). We were in the R44 at a quiet uncontrolled airport (6B6) in Maskachusetts doing some pattern work (take off, fly around in a circle, land). I stressed the importance of checking fuel levels and temperatures/pressures on downwind (flying at 500′ above the ground parallel to the runway) and before lifting up or taking off. I did the standard Robinson flight instructor trick of pulling the “gages” circuit breaker (aeronautical engineers can’t spell?). This causes the analog gauges to show 0 fuel, 0 oil pressure, and 0 oil temperature. The pilot flew 3 or 4 patterns without noticing anything amiss (i.e., missed at least checks of the gauges). His pilot friend in the back seat also didn’t notice anything. I asked them what they thought the most common cause of aircraft engine stoppage was. “Cylinder heat temperature too high?” was the answer (supposedly it is fuel exhaustion). I reminded them to make sure to do the pre-take-off and downwind checks. We fly at least 4 additional patterns without anyone noticing a problem. I asked the flying pilot to set the helicopter down on the runway and explicitly asked “How do the gauges look?” He responded, “fine”. His friend in the back seat agreed that nothing was amiss. Nervous myself about the fuel situation, I pushed the breaker back in, but not before noting that 0 fuel wasn’t a great way to fly. (In retrospect, I didn’t have to be nervous because Robinson’s orthographically-challenged engineers wisely put the low fuel light (10 minutes) on a separate circuit.)

Update: I’ve now seen some video, thanks to reader comments, in which the rotor system, still attached to part of the transmission, is also separated from both the tail and the fuselage. That’s casts some doubt on the above theory, but I will leave it in place as a reminder of how wrong I usually am.

25 thoughts on “Tailboom chop in the New York City tour helicopter crash”

  1. Surprised a turbine ranger would stall as quickly as a piston Greenspun-44. Piston copters are really sketchy. The 2nd video showed the blades intact. Blanco said manetenance issue.

    Reply

    • lion: an airfoil stall is not the same thing as an engine stall. You can stall an airplane wing or a helicopter main rotor system even with a perfect running engine at 100% power.

  2. The scenario you describe seems very plausible to me. Of course, it could have been something else, but it’s likely that those helicopters operate with minimal fuel onboard. I was biking on the Manhattan West Side bike path about 20 minutes before the tragedy and only found out about it when I got home.

    Reply

    • Yes. If you’re going to load four adult tourists and one pilot into a B206B you will be over max gross weight with full fuel.

  3. I know very little about helicopters and even less about their maintenance but I wonder if this transmission issue from last September might have been significant?
    ——- SDR 1 ——-
    Date Of Report 2024-09-24 Operator Control Number MHR22024092400733
    Date Of Occurrence 2024-09-24 Aircraft Registration 216MH
    Aircraft Manufacturer BELL Aircraft Group Code 206L4
    Aircraft Manufacturer Model 206L4 Aircraft Make Model Sequence
    Air Carrier Name Unknown Air Carrier Operation Code
    Operation Type G Stage Of Operation CRUISE
    Severity Factor Air Transport Association Code Main Rotor Gearbox
    Precautionary Procedures OTHER Nature Of Condition WARNING INDICATION
    Segment Descriptive Name Of Part BEARING
    Defective Location On Aircraft TRANSMISSION Failed Part Condition FLAKING
    Part Total Time 188 Part Total Time Since Overhaul
    Component Manufacturer Component Manufacturer Model
    Component Manufacturer Number Manufacturer Part Number TAI6040032105
    Ata Code
    Remarks TRANSMISSION ASSEMBLY HAD METAL IN OIL.

    Reply

    • If the transmission fails that operates more or less the same as an engine failure and it should still be possible to autorotate.

    • It should be possible to recover from complete loss of a tail rotor by entering an autorotation (remove the torque and you don’t need the anti-torque system). Loss of a tail rotor at cruising speed doesn’t typically result in a dramatic spin because the vertical stabilizer resists spinning. Even if a helicopter spins due to tail rotor failure there shouldn’t be a boom chop.

    • Wow, anon…that video seems to suggest something catastrophic happened and not running out of fuel?? Phil, what say you when you see that video?

    • Hmm… without a tail rotor the helicopter will spin due to the engine torque. But if there is no engine power there is no torque and no reason for the helicopter fuselage to spin.

    • Not that I’m aware of. A real-world autorotation happens about 1/100,000th as often as a training auto.

  8. Wow, anon…that video seems to suggest something catastrophic happened and not running out of fuel?? Phil, what say you when you see that video?

    Reply

    • I can’t interpret it well enough to see whether it shows the helicopter beginning to descend (indicative of an engine failure and, if no autorotation is entered, a blade stall) before the tailboom chop. Running out of fuel isn’t catastrophic in and of itself, but it can become catastrophic within seconds if the pilot doesn’t enter an auto.

    • It’s when a helicopter glides back down to earth without the engine running. The pilot has to “fall” fast enough to keep the rotor spinning, then just before landing the pilot “pulls up” to slow down and land softly.

    • Thanks for the video. I had thought that if the main gearbox seizes the heavy rotor system is supposed to cause a shaft to shear and leave the rotor system still free to rotate (and autorotate), contrary to what the jet pilot says in the video. The engineers design for that possibility, in other words. But here’s an accident analysis video where a gearbox failure leads to rotor separation on a Airbus: https://www.youtube.com/watch?v=zPX7NJe1Mog&t=19s

    • Thanks for the video! Could fuel starvation in flight actually damage any moving parts? I’m guessing not, but I don’t know much about helicopter systems (correction, I know nothing).

    • Fuel starvation doesn’t damage anything and neither does low g. It is the pilot’s action or lack of action after either event that causes disaster.

    • Guards and visitors at the Museum of Fine Arts today. Pedestrians outdoors in Brookline yesterday. The Science remains strong.

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