What Happens When a Helicopter’s Engine Fails?

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Autorotation above Welshpool Airfield

Ready to start autorotation practise? Image by Helen Krasner

Many people think that if the engine fails in a helicopter, you’re doomed!

They assume that the aircraft will simply fall out of the sky like a stone, and that the pilot will be utterly helpless.

In fact, I have even seen this scenario described in a bestselling book by an author who should know better.

But nothing could be further from the truth.

If the helicopter engine stops, the experienced pilot simply puts the machine into a controlled descent which allows air from below to turn the rotors.  This is known as ‘autorotation.’

The pilot can then manoeuvre the helicopter to a safe landing site, and so long as he or she is reasonably skilled, this will not be a crash site.  In fact, everyone in the aircraft ought to be able to walk away safely, and the machine should even be in one piece too.

So how does autorotation work?

Air From Below Keeps the Rotor Blades Turning

In normal flight, the engine drives the rotor.  But it is not the engine which actually keeps the helicopter flying.  It is the air flowing over the turning blades which produces lift, and it is this lift which keeps the helicopter in the air.  So if the engine stops – actually, a very unlikely event with modern helicopters – we just need another way to produce lift.

If a descent begins immediately after the engine stops, the airflow from below will keep the blades rotating.  This might be a little hard to visualize, but it works a little like a windmill, or a descending sycamore leaf.  In any event, it works!  The helicopter will descend, but it will do so at a steady pace, and its direction can be controlled.  It can then be manoeuvred into wind and landed safely.

Student pilots practise autorotations frequently. Image by Helen Krasner

Successful Autorotation: The Pilot’s Role

If the helicopter engine stops, everything will go quiet in the cockpit, and the machine will yaw violently to one side.

When this happens, the pilot needs to instantly lower the collective, taking the pitch off the rotor blades, and initiating a descent.  This is the most important thing, as it is crucial to maintain rotor RPM, ie keep the blades turning.

He or she also needs to keep the aircraft straight with the pedals, and make sure the helicopter’s nose does not drop.

These are the only things which need to be done rather quickly.  Once the helicopter is established in autorotation, it will be descending at around 1,700 feet per minute, which means at a normal flying height of around 2,000 feet there is over a minute to sort things out – not a lot of time, but enough.

Landing Without an Engine

The pilot now looks for a safe landing site – a flat area where he can land into wind.  A flat field without crops or other obstacles is best, but in a city a playing field will do, or even a flat roof!   At this point, the pilot puts out an emergency or ‘Mayday’ radio call if possible.  At about 40 feet above the ground, he starts to raise the nose of the helicopter by pulling back on the cyclic. This slows the helicopter down, and his aim is for it to be level and moving slowly when just a few feet above the ground. At this point the collective is raised, using the last available pitch of the blades to produce lift and cushion the landing. Ideally the landing is normal and gentle, but helicopter skids are designed to absorb shock and thereby cushion a less-than-perfect autorotative landing.

Autorotative Landings

Autorotative landings are not easy for the beginner.  However, prospective pilots practice this exercise many, many times during the PPL(H) course, and regularly afterwards.  It is rare for any pilot to have to do an autorotation in earnest, but I have known some who have, and they have all walked away safely. If you know how to fly a helicopter, engine failure is an emergency, but not necessarily a disaster.

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