Any engineer that has ever gone through helicopter training has wondered “Why doesn’t the collective lower itself after the engine quits?” (see “Teaching Autorotations” for an introduction to why this is important; see also Wikipedia) A typical turbojet-powered aircraft, e.g., Boeing 737 or business jet, resists pilot attempts to stall the machine via a stick pusher. If the airplane is going too slowly, roughly 150 lbs. of force are applied in the nose-down direction to ensure that airspeed is maintained (high-performance airplanes are not easy to recovery from a stall/spin). If rotor speed, and therefore airspeed of the spinning wing, is falling there is one sensible flight control action that is almost always helpful: lower the collective.
Somehow I missed that it actually can be done automatically. The Helitrak Collective Pull Down (follow the link for a bunch of videos) seems to have been available for about 1.5 years for about $15,000 in an R44. Some excerpts:
The low rotor RPM warning signal triggers the device to pull the collective down in less than half a second, eliminating pilot recognition and reaction times. … ultimately, can anyone put a price on the life of a pilot and passengers?
According to the Flight Safety Foundation, pilots take, on average, 2-3 seconds to recognize that a problem is occurring and then another 4-6 seconds to react.
A system that also pulled the cyclic back a little would be yet better, but it would be nice to see this on every helicopter!
Two things philg…
(1) You don’t have stick pushers on the 737 (or most any wing engined aircrafts for that matter). You have it only on the tail engined designs like the DC-9/MD-80/ERJ, etc with a T-tail. What happens is that the T-tail puts the tail in the main wing’s wash at a high angle of attack near stall speeds. This means that when you are about to stall out you cannot correct the condition by lowering the nose because your elevators on the tail don’t work. The stick pusher lets the computer push the nose down before you get into the “dead zone”. You can actually fight it, so it is more of a friendly reminder than an override of controls.
(2) It’s different for a helicopter.A helicopter cannot glide; the rotor is the only thing keeping it aloft. The only reason you want to lower the collective is that you want to keep the rotors spinning longer after the engine quits. But lowering the collective all the way also means you get zero lift and the helo will drop like a rock. It is always a balance between slowing the descent and keeping the potential to slow descent. It is way more complicated than a simple collective pusher can accomplish. You better hope the pilot knows WTF he is doing, because otherwise you are screwed. At some altitudes with some terrain you are screwed even if he is the best pilot in the qworld.
dwight: I sure hope that a helicopter can glide! That’s what we are training students to do. And https://en.wikipedia.org/wiki/Jean_Boulet glided down from 40,820′ in his altitude-record-setting Eurocopter (an Aerospatiale at the time).
I’m not typed in the B737 so I just assumed it had a pusher like most bizjets or the CRJ that I flew for Delta.
https://www.pprune.org/archive/index.php/t-69058.html (where the airline pilots hang out) suggest that the story is complicated for the B737. It seems that it some countries it has been equipped with a pusher. In others with a “nudger”.
Thanks for the correction. I learned something about the Boeings (but maybe the most important lesson is to fly Airbus instead!).
Figured it was the same reason every copter doesn’t have flight by wire. More weight. More money. More maintenance. Longer checklist.
I have a question for Philip (and the other pilots here):
It seems like we have managed to build drones which are both pretty sophisticated and have absurdly simple interfaces. People with minimal training are able to fly drones around with few issues. Would it be possible to totally re-imagine the helicopter interface, full incorporating the various sensors, feedback systems and so forth which keep a drone level and simply operated? Is there something fundamentally different about a helicoper and say a fancy quadcopter drone?
This may seem like a dumb question but is relevant for most of us whose experience is limited to very small aircraft, which don’t have stick pushers.
Does this make landing stick pusher aircraft much trickier than normal or is there an option to turn it off for landing? If not, it would seem there is no more option of coming in a little hot and bleeding off speed once over the runway with the nose temporarily up a bit – this option reduces the possibility that a headwind gust could cause an unintended stall just short of the runway start. Landing very small aircraft, after all, is a process of slowly stalling it when you are just above the tarmac and just above the stall speed. Only after you are rolling down the runway do you pull up the flaps.
GC: There are no dumb questions, only dumb pilots! (i.e., 100% of us)
With heavier airplanes you tend to “fly it on” and don’t get close to a full stall (where the shaker-then-pusher would activate) before the weight-on-wheels sensor disables the system.
The Pilatus PC-12 is a bit of an exception because it can be flown so slowly and landed on such short fields. It has truly nasty stall/spin characteristics so it has a pusher. Pilots who want to extra the maximum out of a short-field landing will have their fingers on or near the pusher disconnect switch on the yoke. This prevents the pusher from slamming the nose wheel down on the runway in the event that the pilot nearly stalls the plane just before the mains touch. I have gotten the shaker on a few landings, but never the pusher.
I think that there are some more sophisticated planes that use radar altimeter as an input to the pusher system so that it isn’t activate right near the ground.
Not certain where or how the “Flight Safety Foundation” obtained their data on response times for pushing the collective down after a helo pilot realizes the engine is out or an other emergency that requires an immediate autorotation entry, but if it’s truly eight or nine seconds on average for total response time, I doubt there is a light helicopter that could be safely landed after an eight or nine second interval for getting the collective down.
I do not believe anyone taking that long to get the collective down in an R22 or R44 after engine failure would make it out alive. The heli blades would be stalled and the ship wouldn’t be recoverable.
This invention by Helitrak seems only available for the R22 or R44 and it seems to be a great addition…if you think a derated Lycoming engine in a Robinson is a failure waiting to happen.
A more interesting stat is how many engine failures occur in Robinson Helicopters? Not many, I’d bet.