Malaysian Helicopter Crash

[cpt]

At the reading of the accident report, I am surprised to see that the sudden loss of the left horizontal stabilizer resulted in such a violent pitch down.
This incident already has happened few times on AS 365 N family and always a recovery was possible.
I myself experimented it when air testing a 365N2. I was in a +/- 15° dive, looking for a vne when it happened. I noticed slight oscillations in pitch, pretty much as when we encounter turbulences but in an other hand, it took a much longer time to recover from the vne dive. Might the cause of the violent pitch down of this ill fated helicopter, be an aerodynamic stall of the right remaing half stabilizer ?
As for the hydraulic leakage apparently on the left landing gear bay area, I suppose it could have come from a leak of the 2 liters emergency landing gear extension tank. This typically leaves no indication on the caution warning panel and doesn't affect the flight controls.

[FH1100 Pilot]

Maybe the hydraulic fluid seen by the pilot was brake fluid?

After reading the report, I sort of thought that the left horizontal stabilizer endplate went through the fenestron?

[henra]

Originally Posted by cpt

Might the cause of the violent pitch down of this ill fated helicopter, be an aerodynamic stall of the right remaing half stabilizer ?

Indeed a bit strange that the 'simple' loss of one HS should lead to such a dramatic result.
Maybe it didn't simply fall off but twisted in a Nose Down direction. This could also possibly explain the massive excursion in Roll that occurred simultaneously?
At vne the Fenestron 'eating' the endplate should not cause massive Nose Down or Roll.
Still a bit mysterious...

[krypton_john]

... nothing in the preliminary report suggests that anything went through the fenestron though? It only mentions high speed impact damage to the casing.

[Ascend Charlie]

At the reading of the accident report, I am surprised to see that the sudden loss of the left horizontal stabilizer resulted in such a violent pitch down.

The horizontal stabiliser is designed to produce a downforce in flight - helps keep the fuselage level. The two halves are often set at different pitch angles to generate a roll force to counteract the imbalance from a low-set tail rotor.

So, one side breaks off. Suddenly you have lost 20 or 30kg from 10' or 15' behind the cg - pitch down happens.
You lose the downforce from that side - pitch down happens.
You lose some of the roll balancing force - roll happens.

Add these together, 5h1t happens. Why did the owner succumb to the pressure to keep going without looking at the damage? It is pretty common for a wheeled helo to sink into soft ground and dislodge the brake hose lines.

[[email protected]]

Maybe the hydraulic fluid seen by the pilot was brake fluid?

yes, much more likely than the emergency gear reservoir which is quite well protected, unlike the brake fluid pipes which could easily be damaged during such a landing.

The pitch and roll after losing the horizontal stab isn't a surprise, but the fact he didn't recover it quickly is - that alone shouldn't have resulted in a fatal crash.

[cpt]

To Ascend Charlie,

The horizontal stabilizer on the N2/N3, is made of one block of very thin carbon fiber and there is no difference in pitch setting along its span to counterbalance the "effect of low tail rotor" (not even on other helicopter types I know)
Once again this inflight left stabilizer rupture already happened to myself and some others. In my case despite your theory, I barely noticed a difference (just a few oscillations in pitch and nothing in roll) although I was nearing the vne.

To crab,

I agree that given the circumstances, the hydraulic leak was more likely due to a torn off brake block. But a leak on this emergency reservoir also happened at several occasions (not under strain but typically from the weldings) Both of these leaks may dump something like 2 liters of hydraulic fluid or slightly more.

I'm just wondering wether the sudden initial and brief nose down after the loss of the left half stabilizer did not initiate an aerodynamic stall (sudden increase of the angle of attack on the upper surface) This aerodynamic stall would suddenly have cut the remaining downward lift and greatly amplified the nose down motion.

[[email protected]]

CPT - even with both horizontal stabs not working (one missing and the other stalled) it should still be possible to recover unless you are close to the ground.

[cpt]

Yes crab, it is more or less what I'm saying. My idea about horizontal stabilizer stall is coming from the SA 365C model. On this helicopter when correcting a nose up upset, an overeactive pitch down autopilot control action (without pilot's correction) could possibly stall the horizontal stabilizer. The resulting sharp nose down attitude was said to be critically recoverable below 500' AGL.
Although I've never heard something alike on 365N series, I was thinking that perhaps this could happen with only 50% stabilizer surface remaining.... just a guess ... helicopter aerodynamics are more esotheric than something else.

BTW I know an other type of helicopter (Lama SA315) that crashed after too much pitching nose down without the horizontal stabilizer installed. The pilot was very experienced but never could recover from a loooong dive (more than 500') in mountain flight.

[Never Fretter]

Looks like last month's discussion was based on only the preliminary report. The final report came out last week: Final Report: AS365N3 9M-IGB Fatal Accident