San Andreas is an awful movie, but given how helicopter-centric its plot is I'm wondering if they even got the autorotation landing correct? In the middle of the movie the hero is cruising in (I think) a Bell 412 when the engines fail. He tells his passenger they're going to autorotate, and he proceeds to do the following maneuver three times: Precipitous pitch down, then after picking up a lot of speed, a flare. (The final "landing" flare is attempted in a crowded parking lot where he tries a last-second evasive maneuver that destroys the helicopter.)

My understanding is that a proper autorotation always simply consists of pitching to a specified glide path and not changing pitch until the landing flare. Do any conditions or rotary-wing vehicles call for a different autorotation maneuver?

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    $\begingroup$ I can't find a clip online. If he dives immediately, then it's unrealistic. If he dives after entering autorotation, and the airspeed is low, then it could be realistic. There is nothing stopping you diving (increases airspeed and rate of descent whilst reducing rotor speed) and flaring (does the opposite) as many times as you want but diving and flaring reduces distance travelled and time in the air since it's all about converting one form of energy into another. As you say, you would normally select pitch for max range or minimum descent rate, leave it there and flare at the bottom. $\endgroup$ – Simon Nov 29 '15 at 19:44
  • $\begingroup$ If your first action is to pitch forward, then you die. The rotor speed will drop to an unrecoverable speed very quickly. $\endgroup$ – Simon Nov 29 '15 at 19:46
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    $\begingroup$ I found it. Complete nonsense. Read this $\endgroup$ – Simon Nov 29 '15 at 19:51
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    $\begingroup$ @JPhi1618 - No kidding. Most of the movie required extreme suspension of disbelief, logic, physics, etc. I guess I had hoped that since they obviously had access to real helicopters and pilots, and somehow picked up the word "autorotate," maybe they'd at least ask someone to demonstrate a realistic autorotation in the helicopter they were filming. Maybe they did and decided it just wasn't exciting enough for audiences.... $\endgroup$ – feetwet Nov 30 '15 at 16:22
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    $\begingroup$ Heh, yea. That's ok - if the helicopter was depicted as poorly as computers are in movies, it would fly with the main rotor facing forward and travel at about 800mph before kicking in the turbo boost. $\endgroup$ – JPhi1618 Nov 30 '15 at 16:26

Let's see how many things we can find wrong.

  1. There's a bang, smoke starts coming from the exhaust. Master caution comes on and his first response is to tap and check the 115v AC gauge! The pilot takes his hand off the collective!

  2. You hear the engine wind down and the rotor RPM drop. He announces a gearbox failure. So why did the engine wind down? His passenger even asks if he switched the engine off. If the gearbox had seized, the engine would have been destroyed or, the input shaft would shear and the engine RPM would increase leading to a very rapid overspeed. There is no gearbox failure mode that results in the engine winding down.

  3. He had a long think and a bit of a chat after the engine and RRPM wind down. In a Bell 412, with the speed of that wind down, the blades would have stalled after 2 or 3 seconds if he hadn't dropped the collective.

  4. He announces that he is going to autorotate. His passenger asks "auto what?" He says we're going to crash. The whole point of autorotation is that you don't crash.

  5. With the RPM already low (let's forget that he would have stalled by now), his first action is to dive and you hear the RRPM increase. In reality, they would go even lower. If he wasn't already stalled (which of course he would have been) he would now.

  6. He was straight and level, with significant airspeed over level open ground. At the first bang and sound of the engine winding down, he would have lowered the collective and flared to preserve RRPM. He would then look for a good place to land, not a shopping centre car park, and fly towards it.

  7. Each time he flares, he pitches forward again and the RRPM drop rapidly. Flaring like that in auto would cause a large increase in RRPM, possibly overspeeding and lowering the nose to the level would cause a small drop in RRPM. Diving again would reduce the RRPM. So why would you flare to increase, then dive to decrease?

In short, the sequence is complete nonsense.

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    $\begingroup$ Excellent narration on everything wrong! But per your first comment on the question: If one wanted to reduce distance traveled to land (e.g., one happens to be exactly over a beautiful landing spot during power failure) would a series of dives and flares ever make sense? Or can you just flare to stop forward motion and then set the collective for a constant descent rate, floating straight down and yanking the collective at the end for a gentle landing? $\endgroup$ – feetwet Nov 29 '15 at 20:58
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    $\begingroup$ Or what's the helicopter analog to the fixed-wing landing mantra "throttle for altitude, pitch for speed?" $\endgroup$ – feetwet Nov 29 '15 at 21:06
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    $\begingroup$ @feetwet A series of dives and flares makes no sense ever. If you are directly over the only safe place to land, then you have a few choices, depending on height. If you have altitude to play with, you could perform a zero speed auto until about 3-400 feet, then pitch down to increase speed (you MUST have airspeed for the flare at the bottom) and use S turns to reduce distance over ground. You could even start by going backwards so that you fly away from the chosen spot, then fly forward to gain speed for the flare. $\endgroup$ – Simon Nov 29 '15 at 21:24
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    $\begingroup$ The easiest, safest option is to simply S turn all the way down, maintaining airspeed and using the collective to keep RRPM under control. If you try to go all the way down with no airspeed, you will arrive close to the ground with a high rate of descent and pulling enough collective to arrest that rate will stall the blades while you are still a long way up and you will fall 50 feet or more to the ground. Airspeed at the bottom is life. Not sure I understand your question about the analog for "throttle for altitude, pitch for speed". $\endgroup$ – Simon Nov 29 '15 at 21:26
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    $\begingroup$ Pitch for speed yes, but you have no throttle (the engine is dead!) and no altitude control. You ]control rate of descent somewhat but you are basically going down fast. At 1000 feet, most non-helicopter pilots are surprised at how close the landing spot is (it's somewhere visible in the chin windows, not out in front) and how quickly you are on the ground (less the 20 seconds). From 1000 feet, at max range airspeed, you are going to be on the ground in less than a mile. A key point is that you extend the glide with airspeed, not with collective which is only used to control RRPM. $\endgroup$ – Simon Nov 29 '15 at 21:39

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