Why do military helicopters crash rather than autorotate?
 

Hi,

This is a question I have wanted to ask for a good while. This forum isn't specifically related to military flying, but hopefully some of you guys have knowledge about that.

A while ago, I was watching black hawk down. In the movie, blackhawks get shot down at low altitude and crash. Crew members die. It is based off of real life and similar crashes happen often. I also heard that if the tail rotor on something like an AH-64 gets shot and malfunctions, the helicopter will crash.


My question is, why can't the pilots autorotate down to a fairly soft landing? I know in the R-22 they say if there is a tail rotor problem, enter an autorotation. Why would a helicopter have to crash because of just a tail rotor failure?

Thank you in advance for help

Ifresh, in theory you may well be able to autorotate with a tail rotor failure if the aircraft gets it at a convenient time. "Convenient" would normally mean at an altitude which gives you time to think, at cruise speed to help maintain directional control, in a lightweight aircraft, using low power, over a nice big soft field.
I regularly teach this malfunction in the simulator in a benign environment, and even then most people will crash the first couple of times.
A heavy aircraft full of troops at low altitude over a built up area with people shooting at you is the worst environment I can imagine to get a tail rotor failure. Barring a miracle, it is always going to end in a world of hurt. When you are hovering at heavy weights, the engines are producing a huge amount of torque. If the tail rotor departs, there is no anti-torque reaction and you will start to spin rapidly. Its inconceivable that you could turn the engines off and lower the collective quickly enough to maintain control. Even if you can, its only going to help if you are over a nice big flat level area with no bad guys in it.

Besides which, military rotorcraft no longer crash: they only suffer 'hard landings.' Which probably reassures Joe Public and the beancounters more than the grunts in the back. :E

I/C

high disk loading

There are a lot of things around a tail rotor that can cause enormous trouble if the get damaged. A drive failiure is the standard emergency procedure but that is easy. More important things that can go wrong are:

Severe blade damage. This can cause the a tailrotor blade to fly so far out of track it hits the pylon or causes such an imbalance that the tail rotor assembly is torn from its mountings. Whether it is restrained by the controls or not it will just be a chunk of metal thrashing around the back and a conventional autorotation will be unlikely.
Control wire damage. The control cables on all aircraft are tensioned so to avoid any slack in their operation. 2nd World War gun cine film demonstrates an untold number of aircraft that suddenly go into an uncontrollable roll when one of the aileron cables is severed. The same effect will cause a helicopter to spin uncontrollably if a tail rotor cable is cut. Some helicopters have a centering device to prevent this if this happens;---- unless the device itself is hit.
Ground Proximity. I know of one case with a Puma where they had a tail rotor failiure in the hover and managed to shut down the engines and carry out a controllable landing. However, that manoeuver had been practised in a simulator; without that training they would have be unlikely to get the same result.
Low level with high speed you have a multitude of problems where you may just, if you are lucky, get away with it.

That cannot be forcast.

Hollywood also has an affect too. Lots of smoke and beeping followed by "terrain terrain" is far more cinematic that a nice entered auto and landing.

Drama.. it's all in the drama.

I forgot that. The seat cushions in helicopters are highly inflammable and wherever the helicopter is hit the seats catch fire and clouds of smoke comes out of the cabin.

Don't believe much of what you see in the movies. They play by movie rules, which have little, if anything, to do with real life. That said, when people are shooting at helicopters with high explosives and incendiary rounds, bad things can happen, and lots more than just the tail rotor can be damaged or blown completely away. There are lots of things on military helicopters that are far more flammable than seat cushions, including fuel and ammunition among others.

Just joking
 

Open Joke"
Autorotation is not the solution to every problem. Would that it were it would make life so much easier.:E
I know of one bright lowtime lad who - having the engine chip light come on - rolled the throttle smoothly to idle and Auto'd to a perfect landing in a very, very small swamp amongst the trees - 1 mile from a very nice field with farms, farmer's daughters, telephones and roads and all that good stuff.
Sometimes, however, there is only time to call "#$%^ brace, brace, brace!" and hang on for dear life.:ooh:
"Close Joke

Thanks for the replies

Btw, the movie is based off real life

I think you'll find most people here know that! ;)

at such low level, and high weight, it would be near impossible to enter an auto rotation.

In Michael Durant's autobiography, he says that when the tail rotor failed, the helicopter started spinning so quickly that the centrifugal force made it impossible for the co-pilot to cut the throttles almost immediately.

often its the "dead mans curve" that mil helicopters tend to operate in or around making autorotation impossible no matter the damage etc

Height-velocity diagram - Wikipedia, the free encyclopedia

Like this one?
http://www.thedenverchannel.com/2010...12_640X480.jpg
This photo, from a source, shows the wreckage of the Apache helicopter on the side of a mountain near Pikes Peak. The Army referred to this as a "hard landing."
Army: Helicopter Crash On Pikes Peak Caused $26M In Damage - Denver News Story - KMGH Denver

Gotta love their definition of a "hard landing"...

High disk loading primarily caused by operating at heavy op weights on the edge of the envelope, with external inputs at very inopportune times in difficult terrain/surroundings.

Of course, RPG and small arms passing through the cockpit can provide a distraction at the time too.

Hollywood portrayal of such events is bolleaux. Most of the incidents you refer to are during the critical phases of an operation and involve putting the aircraft and crew in situations you'd rather not. However, crews take as many precautions as they can and actively minimise the time spent in the more hazardous areas of the curve.

Most of it is down to luck after that. Can't comment on tail rotors, don't like them, dont have one.

How true. It seems whenever they show a pilot flying a 206, he's usually in the left seat. And the sound is usually from a Bell 47 except in Blue Thunder and a few others.

Some would say it's because they have military pilots in control. :sad:

And don't forget that, whatever the emergency, however minor or major (makes no difference anyway, all the CWP captions will be on and a overly loud warning horn will be sounding like an hyperactive foghorn), the single Immediate Action is to grab the cyclic in both hands and wrestle with it, and make no attempt at all to fly the aircraft or do any drills. Spinning round and round a few feet above the ground for longer that you might think strictly necessary is in the Subsequent Actions.

Also, refer to this scene from the movie "the day after tomorrow" for a highly realistic depiction of a typical in-flight emergency:

thoughts on this
 

you're at 100 feet agl flying at a low airspeed in a very hot environment in a heavy UH60. Lots of power being pulled already. a shocking blast runs through the aircraft. immediately it begins to spin violently because the tailrotor has stopped at a high power setting. you pause however briefly to try and assess what has happened and what to do about it. because you are spinning violently, it is difficult to get your hand on the power control levers, pull them to idle and the get them past the idle detent to off. In the mean time the pilot on the controls has lowered the collective to save rotor rpm so you are descending at a rate of several hundred fpm. Also since the blast hit your tail area, possibly a large amount of weight relative to your W and B has been removed from the aircraft causing the nose to drop violently due to the out of balance condition. in the meantime since you are over a hostile area and trying to locate a landing area amongst buildings, cars etc...

its easy really