# Why don't helicopters ever break down and fall out of the sky? **URL:** \ **Category:** Factual Questions\ **Created:** [November 15, 2009, 5:31pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668 "2009-11-15T17:31:12Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 7:24pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/21 "2009-11-15T19:24:33Z")
Here are some videos of autorotations: [0-Speed Autorotation from 100-200 feet, R22 helicopter](http://www.youtube.com/watch?v=jH03stFao4k) [R-44 Full Down Auto](http://www.youtube.com/watch?v=vUzoUWvDXjU&feature=related) [Helicopter in autorotation to ground](http://www.youtube.com/watch?v=rlEBHLlQrfY&feature=related). The horn you hear is the low rotor RPM warning horn. [Full down autorotaions](http://www.youtube.com/watch?v=gaE7SAmQJ3A) [0 Knots 1000ft Autorrotation R22](http://www.youtube.com/watch?v=FvVTt1ib5js&feature=related) It’s a little hard to see on the last one, but the instrument in the upper-right corner of the panel is the tach. The Robbo uses intersecting needles showing the percentage of engine RPM and rotor RPM. If you look carefully, you can see that the engine RPM needle (on the left of the gauge) is at idle, and the rotor RPM is near the top. The warning horn sounds a couple of times, but it’s designed to warn the pilot before the rotor RPM gets _too_ low. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [November 15, 2009, 7:25pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/22 "2009-11-15T19:25:14Z")
> [@Johnny\_L.A](#): > > Tail rotor failure? I presume loss of tail rotor would always be catastrophic? Is there anything to get round this circumstance? Also, I speculate that anything that locked up the rotors would be Bad News indeed, unlike on a plane where the glide apparatus comprises some big static surfaces. Can a pilot disabuse me of the above layman’s fears? ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [November 15, 2009, 7:33pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/23 "2009-11-15T19:33:24Z")
> [@jjimm](#): > > I presume loss of tail rotor would always be catastrophic? Is there anything to get round this circumstance? I’ll refer this to the helo pilots. I’m not aware of any “save” for a true tail rotor failure. Perhaps twin rotors that rotate in opposite directions would allow for operations without tail rotors, but with a single rotor system I think the tail rotor is essential for stability and control. > [@](#): > > Also, I speculate that anything that locked up the rotors would be Bad News indeed, unlike on a plane where the glide apparatus comprises some big static surfaces. Correct. The spinning of the rotors is what generates the lift. No spin, no lift. If the rotors stop there is nothing left to hold you up, the drag generated by the helicopter structure will not significantly slow your descent. You will hit the ground hard. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 7:34pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/24 "2009-11-15T19:34:53Z")
> [@jjimm](#): > > I presume loss of tail rotor would always be catastrophic? Is there anything to get round this circumstance? > > Also, I speculate that anything that locked up the rotors would be Bad News indeed, unlike on a plane where the glide apparatus comprises some big static surfaces. > > Can a pilot disabuse me of the above layman’s fears? Tail rotor failures are nothing to sneeze at. But what the tail rotor is, is the anti-torque rotor. If it fails, just remove the torque. That is, chop the power and enter autorotation. You might get some yaw just because of friction, but that’s OK. It is also possible to extend your glide by carefully applying power. This will allow you to go a greater distance, but will also introduce yaw. In any case, just enter autorotation and be sure to hold the throttle closed when you flare. I’ve never heard of rotors ‘locking up’, unless someone posted an instance in another thread once. Not something I’d worry about. One big killer in helicopters (other than things like wire strikes) is low rotor RPM. I saw a video of a helicopter that ‘just fell out of the sky’ when the pilot failed to maintain proper rotor RPM. Low rotor RPM (letting the rotors get too slow) or retreating blade stall (flying too fast, so the retreating blade loses lift) can result in mast-bumping or blowback. Either can cause a rotor blade to slice into the boom (‘boom chop’). The R22 manual appendix notes use the word ‘doomed’. Mast-bumping may also cause damage such that the rotor system departs the aircraft. I hasten to add that these situations are avoided by proper training and technique. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 7:57pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/25 "2009-11-15T19:57:06Z")
> [@Johnny\_L.A](#): > > The R22 manual appendix notes use the word ‘doomed’. Ah. Here it is (edited to fix pdf-to-html scan errors and to add emphasis to ‘doomed’ line – [.pdf version](http://74.125.155.132/search?q=cache:uEhh9DuNtRQJ:ihst.rotor.com/portals/54/jhsat/accident_reports/216386.pdf)): > [@](#): > > R22 Helicopter Safety Notice SN-24 > > To: Date: 8 September 1986 All R22 Owners, Dealers and Pilot Operating Handbook Subscribers > > LOW RPM ROTOR STALL CAN BE FATAL > > Rotor stall due to low RPM is still involved in more helicopter accidents, both fatal and non-fatal, than any other contributing factor. Frequently misunderstood, rotor stall is not to be confused with retreating tip stall which occurs only at high forward speeds when stall occurs over a small portion ofthe retreating blade tip. Retreating tip stall causes vibration and control problems, but the rotor is still very capable of providing sufficient lift to supportthe weight of the helicopter. Retreating tip stall has not been a problem with the R22. > > Rotor stall, on the other hand, can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky. Fortunately, rotor stall most often occurs close to the ground during take-off or landing and the helicopter only falls four or five feet. The helicopter is wrecked but the occupants survive. However, rotor stall also can and does occur at higher altitudes and when it happens at heights above 40 or 50 feet it is most likely to be fatal. > > Rotor stall is very similar to the stall of an airplane wing at low airsqeeds. As the airspeed of an airplane gets lower and lower, the nose-up angle or angle-of-attack of the wing must be higher and higher for the wing to produce the lift required to support the weight of the airplane. At a critical angle, (around 15 degrees or so) the airflow over the wing will separate and stall causing a sudden loss of lift and a very large increase in drag. The pilot recovers by adding power and diving the airplane to recover the lost airspeed > > The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower and lower, the nose-up angle-of-attack of the rotor blades must be higher and higher to generate the lift required to support the weight of the helicopter. Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air through the rotor provides the necessary increase in blade angle-of-attack. Again at a critical angle, as with the airplane wing, the blade airfoil will stall, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing rotor RPM to quickly decrease even more, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades making recovery virtually impossible even with full down collective. > > When the rotor stalls it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first allowing it lo dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes referred to as ‘rotor blow-back’. Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls. Due to the magnitude of the forces invoked and the flexibility of rotor blades, hub flapping stops will not prevent the boom chop. \*\*The resulting boom chop, however, is somewhat academic. as the aircraft and its occupants are already doomed by the stalled rotor before the chop occurs. \*\* > > To prevent rotor stall and its catastrophic results the pilot must always do whatever is required to maintain a safe rotor RPM. It must take precedence over all other considerations, even if it means landing short in a swamp instead of trying to stretch your glide to the dry road beyond. > > Remember the power output of the engine is proporticnal to RPM and when the RPM is low you have less power available from the engine with which to regain the lost RPM. The power-on low RPM recovery procedure of simultaneously rolling on throttle while lowering collective must be practiced until it becomes an automatic reaction to any indication of low RPM. Low airspeeds combined with high sink rates must always be avoided and full collective must never be pulled until the helicopter is within one foot of the ground. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [November 15, 2009, 8:45pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/26 "2009-11-15T20:45:45Z")
> [@Nametag](#): > > The collective pitch control simultaneously changes the angle of attack on all the rotor blades; it normally controls altitude. By lowering the collective, the pilot reduces both lift and drag, immediately starting a descent. The descent causes air to turn the blades, and with the right balance of lift and drag, the rotor keeps turning, and keeps providing lift, and permits a controlled glide. I know it sounds like pushing a sailboat with a fan, but it works. I always figured that was the throttle. I’m not understanding this autorotation thing. Seems like descending once the engine stalls would be like unplugging a small electric fan and blowing in the front of it (against the wind) to keep it going. That’s just going to slow it down and make it spin in reverse. The main rotor pushes air down, so how is going down going to keep it spinning? ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 15, 2009, 9:05pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/27 "2009-11-15T21:05:03Z")
> [@MikeS](#): > > The difference between falling & gliding can be the difference between life & death. As an example, consider [Air Transat 236](http://en.wikipedia.org/wiki/Air_Transat_Flight_236), which ran out of fuel over the Atlantic but was able to glide almost 100 miles to the nearest airport in the Azores. If it had just fallen into the ocean when the fuel ran out, the ending of the story would have been very different indeed. Just kicking some numbers around that doesn’t sound right. It would take a best-glide speed of almost 400 mph which is probably double that of an airliner. Coming off of 34,000 feet I would expect something closer to a 50 mile radius assuming ground level is sea level. I’m guessing there were some serious tail winds or the captain took advantage of some rising currents or both. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 9:05pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/28 "2009-11-15T21:05:37Z")
> [@snailboy](#): > > I always figured that was the throttle. I’m not understanding this autorotation thing. The throttle, which is like a motorcycle throttle, is at the end of the collective lever. I posted a link to Wiki’s explanation of autorotation in post #3, but [this page is better](http://www.cybercom.net/~copters/aero/autorotation.html). Much more thorough, and with more illustrations. > [@snailboy](#): > > The main rotor pushes air down, so how is going down going to keep it spinning? Don’t think of it as a fan pushing air down. Think of it as an airplane wing; only instead of being pulled through the air by the propeller, it is pushed through the air by the engine. Here’s the part that explains in a nutshell why the rotor doesn’t spin backwards: [quote] [ul][li]The _driven region_, also called the _propeller region_, is nearest to the blade tips and normally consists of about 30 percent of the radius. The total aerodynamic force in this region is inclined slightly behind the rotating axis. This results in a drag force which tends to slow the rotation fo the blade.[/li][li]The _driving region_ or _autorotative region_, normally lies between about 25 to 70 percent of the blade radius. Total aerodynamic force in this region is inclined slightly forward of the axis of rotation. This inclination supplies thrust which tends to accelerate the rotation of the blade.[/ul][/li][/quote] So the air coming up through the driving region allows the driving region to generate thrust, which drives the driven region. The driven region’s drag counteracts the driving region’s thrust, but also produces lift. ---
**Author:** ![ivn1188](https://avatars.discourse-cdn.com/v4/letter/i/e480ec/32.png) [@ivn1188](https://boards.straightdope.com/u/ivn1188)\ **Post date:** [November 15, 2009, 9:17pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/29 "2009-11-15T21:17:15Z")
> [@Magiver](#): > > Just kicking some numbers around that doesn’t sound right. It would take a best-glide speed of almost 400 mph which is probably double that of an airliner. Coming off of 34,000 feet I would expect something closer to a 50 mile radius assuming ground level is sea level. I’m guessing there were some serious tail winds or the captain took advantage of some rising currents or both. The Airbus A330 has a cruising speed of about 550mph. The best glide speed is about 220. Here’s the official report: 65 miles. [http://www.moptc.pt/tempfiles/20060608181643moptc.pdf](http://www.moptc.pt/tempfiles/20060608181643moptc.pdf) ---
**Author:** ![Annie-Xmas](https://avatars.discourse-cdn.com/v4/letter/a/ecc23a/32.png) [@Annie-Xmas](https://boards.straightdope.com/u/Annie-Xmas)\ **Post date:** [November 15, 2009, 9:39pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/30 "2009-11-15T21:39:11Z")
A NBC -NY News Channel 4 helicopter crashed in 2004. The pilot managed to land on a roof and not crash in the street, where a police chase was going on. [Here](http://www.nearlygood.com/video/choppercrash.html) is the video Newscopter 7 took. Scarey. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 9:43pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/31 "2009-11-15T21:43:21Z")
> [@Annie-Xmas](#): > > A NBC -NY News Channel 4 helicopter crashed in 2004. The pilot managed to land on a roof and not crash in the street, where a police chase was going on. > > [Here](http://www.nearlygood.com/video/choppercrash.html) is the video Newscopter 7 took. Scarey. That looks like a tail rotor failure (and I think I might have had that confirmed at some point, but I’m not sure). Unfortunately, autorotation does require a place to set down. ---
**Author:** ![aruvqan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aruvqan/32/2891_2.png) [@aruvqan](https://boards.straightdope.com/u/aruvqan)\ **Post date:** [November 15, 2009, 9:54pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/32 "2009-11-15T21:54:05Z")
> [@Johnny\_L.A](#): > > Autorotations (practice ones, anyway) are one of the most fun things you can do in a helicopter. 🙂 Really? Next time Ill get him to do that instead of contour flying … ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 9:56pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/33 "2009-11-15T21:56:43Z")
Contour flying is fun, too; but it was frowned upon by the place I rented. Also fun is landing on a plateau, getting as much speed as you can, as close to the ground as is safe, and flying over the edge. But autos are like a roller coaster. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [November 15, 2009, 10:05pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/34 "2009-11-15T22:05:41Z")
> [@Freudian\_Slit](#): > > So what do airplanes do when they lose power? Glide down…really fast? Isn’t that pretty much the same as falling? Not if it’s done [with style](http://www.youtube.com/watch?v=HD9rrXX0HnI). ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [November 15, 2009, 10:09pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/35 "2009-11-15T22:09:36Z")
Does autorotation work for two-rotor aircraft like the CH-47 Chinook, or the V-22 Osprey? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 10:14pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/36 "2009-11-15T22:14:05Z")
It works for tandem-rotor helicopters (not that I have any experience). I don’t know about the Osprey, but I assume it can. ---
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [November 15, 2009, 10:54pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/37 "2009-11-15T22:54:20Z")
> [@Lumpy](#): > > Does autorotation work for two-rotor aircraft like the CH-47 Chinook, or the V-22 Osprey? I’m pretty sure the Osprey’s rotors are interconnected via a drive shaft. I’ll try to confirm this, but I remember reading that either engine (in case of an engine failure) could drive both rotors sufficient for a semi-normal landing. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [November 15, 2009, 10:59pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/38 "2009-11-15T22:59:47Z")
> [@ivn1188](#): > > The Airbus A330 has a cruising speed of about 550mph. The best glide speed is about 220. > > Here’s the official report: 65 miles. [http://www.moptc.pt/tempfiles/20060608181643moptc.pdf](http://www.moptc.pt/tempfiles/20060608181643moptc.pdf) converted to miles that would be 253 mph or 4.2 miles per minute and the distance of 65 nautical miles is 74 statute miles. if they were descending at 2000 fpm then they had 17 minutes of time from 34,000 feet. 17 minutes times 4.2 miles per minutes equals 71.7 miles which is very close to the 74 miles flown. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2009, 11:19pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/39 "2009-11-15T23:19:32Z")
> [@pullin](#): > > I’m pretty sure the Osprey’s rotors are interconnected via a drive shaft. I’ll try to confirm this, but I remember reading that either engine (in case of an engine failure) could drive both rotors sufficient for a semi-normal landing. Yes, the rotors are interconnected (as they are on tandem helicopters as well). According to [this page](http://www.helicopterpage.com/html/tiltrotor.html), the V-22 Osprey does have freewheeling units for autorotation. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [November 15, 2009, 11:20pm UTC](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668/40 "2009-11-15T23:20:25Z")
> [@Magiver](#): > > Just kicking some numbers around that doesn’t sound right. It would take a best-glide speed of almost 400 mph which is probably double that of an airliner. Coming off of 34,000 feet I would expect something closer to a 50 mile radius assuming ground level is sea level. I’m guessing there were some serious tail winds or the captain took advantage of some rising currents or both. As an aside, the B747 has a glide ratio of about 17:1, that would give them a glide distance of around 100nm from 36000 feet, so those sort of glide distances are not unreasonable. Airliners are very good gliders despite their size. [Previous page](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668.md?page=1) [Next page](https://boards.straightdope.com/t/why-dont-helicopters-ever-break-down-and-fall-out-of-the-sky/517668.md?page=3)