On April 9, the NTSB held a public meeting to discuss the crash of a LifeNet helicopter in Mosby, Mo., on Aug. 26, 2011. The Eurocopter AS350B2 ran out of fuel, according to the NTSB, and the pilot failed “to successfully enter an autorotation when the engine lost power due to fuel exhaustion.” What the pilot did not do, the Board explained, is move the cyclic control aft when the engine failed.
This is an important point that must be emphasized to all helicopter pilots, according to Pete Gillies, chief pilot at Western Helicopters in Rialto, Calif. “It is the mistake that leads to helicopters having terrible accidents,” he told AIN, “where the pilot has lost control of the helicopter because he or she did not instantly get the cyclic back as soon as an engine failure occurred.” Gillies gave a presentation on this subject during the “Autorotations–Reality Exposed” safety seminar at the Heli-Expo show on March 6.
In its investigation of the Mosby accident, the NTSB conducted simulator tests, which it said “showed that when a loss of engine power occurs in the Eurocopter AS350B2 at cruise airspeeds, the pilot must simultaneously apply aft cyclic and down collective in order to maintain rotor rpm and execute a successful autorotation. However, the pilot’s autorotation training was conducted at airspeeds below cruise where less aft cyclic is needed to enter an autorotation. Further, FAA guidance on performing autorotations stresses lowering the collective as the initial step in entering an autorotation, does not emphasize the importance of other flight control inputs, and provides minimal information on the critical entry phase of autorotations.”
Limited Cyclic-back Guidance
The helicopter industry is well aware of cyclic back, but the subject gets little attention, even from the FAA, Gillies asserts. “The FAA does not require any mention of this in flight training programs or manuals,” he said. The NTSB agrees, and in its findings on the Mosby accident wrote, “Because of the lack of information about the entry phase of autorotations in the [FAA’s] Helicopter Flying Handbook, helicopter pilots may not be aware that there are flight conditions in which immediate and simultaneous control inputs, not only lowering collective, are required to enter an autorotation.” The FAA’s helicopter practical test standards (PTS) do not mention the use of cyclic during autorotations but require that the pilot “maintains rotor rpm within normal limits.” The helicopter private pilot PTS were last revised in 2005 and the commercial PTS in 2006.
Another NTSB finding from the Mosby accident mentions the need to move the cyclic back: “Without specific guidance regarding the appropriate control inputs for entering an autorotation at cruise airspeeds, the pilots of helicopters with low-inertia rotor systems may not be aware that aft cyclic must be applied when collective is lowered to maintain control of the helicopter and perform a successful autorotation.”
According to Gillies, “The key to all this is the rotor rpm. When the rotor rpm, for whatever reason, drops to that [low] point with no engine to bring it back up, the rotor rpm continues to decrease and eventually will reach zero and [the rotor blades] will begin turning backwards if the helicopter drive system has a centrifugal clutch or if it’s a free-turbine helicopter where the rotor can turn backwards with the engine stopped.”
The problem, he explained, is that during training helicopter pilots are taught a smooth entry to autorotation. “When we’re taking checkrides we do the best we can to make a smooth coordinated entry so the aircraft goes from normal flight into a nice descent. That is the point–the beginning of the autorotation–where if it is not done the way I’ve described with the cyclic coming back, you’re headed for a severe problem. You lose control of the helicopter and it can never be brought back. Almost all autorotations are begun with the pilot putting the pitch [collective] down smoothly, bringing the cyclic back smoothly and putting the engine at idle. You don’t chop the throttle then expect the student to react. You make a smooth entry. Many pilots think this is the way all autorotations begin.”
In the real world, however, even with high-time pilots and the fanciest of modern avionics, he said, “The pilot has no clue that the engine is going to quit. And it quits. At that point what the pilot should do is instantly bring the cyclic back. I mean now. The mistake is for the pilot to look at the panel and try and verify that something’s wrong with the helicopter and then take action. The [incorrect] action is usually putting the pitch down rapidly and pushing the cyclic forward to gain or maintain airspeed. That is the end of the flight.”
For the rotor blades to keep turning, Gillies explains, air needs to keep moving through the blades from below the helicopter. “You’re not in an autorotation until you’ve got air coming up through the blades. That is the only thing that will build the rotor rpm or hold it in the green. Letting the rotor rpm get too slow is deadly. The flight’s over.”
What happens aerodynamically if the cyclic isn’t brought back and the rotor rpm drops too low, Gillies notes, is that on “one side or the other you have retreating blade stall. So the aircraft starts to turn, and the more the pilot tries to fix it, the harder it turns, because if retreating blade stall is making the aircraft turn one way and the pilot tries to turn the opposite way, he puts even more pitch into retreating blades that are already stalling. So now they stall even more deeply. You have a situation where putting in cyclic to straighten the ship increases the turn. And there’s no way to stop this; there’s nothing you can do. No matter how high the aircraft is when this event happens, when the rotor rpm gets [too low] with nothing to bring it back up, nothing you can do in the cockpit solves the problem. You are a passenger at that point.”
Gillies emphasizes that pulling the cyclic back should be done any time there is a problem. “It may not be an engine failure, it might be something else, a big upset, a big noise, anything. A pilot can’t think about it; cyclic back has to be a reaction, and it has to be done instantly, without any delay. Once you have stabilized the rotor rpm in the green, then, if you have time, you can look at the instrument panel, ascertain if you’ve got an engine out and decide whether or not you need to continue with an autorotation or [do something else].”
Gillies reiterates how to deal with engine failure in a helicopter: “Priority number one: engine quits, cyclic back, pitch down. Priority number two: pick a place to land. Priority number three: make that spot. What you don’t have to worry about is the airspeed. You’re flying a rotary-wing aircraft and rotor rpm is the only thing that counts. Airspeed means nothing. The helicopter will autorotate just fine backwards or sideways, but we’re always emphasizing straight-in or 180-degree autorotations. The real world? Cyclic back, pitch down, pick a place to land. If you have to back up, go sideways, turn around, look this way or that way while you’re coming down, fine. Pick a place to land and don’t miss it.”
Commenting on this issue, Robinson Helicopter chief instructor Tim Tucker described what can happen during an autorotation if the pilot doesn’t move the cyclic control aft. “If you just lower collective and don’t simultaneously add aft cyclic, the nose tucks and drives the rotor rpm down,” he said. “That nose tucking is bad for rotor rpm; it makes the rpm go down, especially if it’s a low-inertia helicopter like the R22, but probably not as much with the R44 or R66. The way we like to do it is to use enough aft cyclic as the collective is lowered to prevent the nose going down. I don’t think we should talk about aft cyclic in an either-or context. I don’t want to say ‘Focus on aft cyclic and not on lowering collective.’ I think it should be done in combination with aft cyclic.”
Tucker, who is on the International Helicopter Safety Team’s Executive Committee, said the organization has sent a draft advisory circular to the FAA that discusses this issue. The draft AC, he said, “discusses that enough aft cyclic should be put in during an autorotation entry to maintain a level attitude.”
Chris Gadbois, owner of flight training company SRT Helicopters in Bakersfield, Calif., agrees that teaching helicopter pilots to move the cyclic aft during autorotations is important, but he also sees this as a standard practice and teaches it to all SRT students. “Entering an autorotation correctly involves lowering the collective and coming aft with the cyclic,” he said, “to slow the aircraft down and [for] loading the rotor disk. As you’re lowering the collective you’re unloading the rotor head. To maintain rotor rpm, come aft with the cyclic to load the rotor back up.”
That said, Gadbois cautions that pilots have to be careful not to overspeed the rotor system when entering an autorotation from a high cruise speed. If a pilot is cruising at 120 knots and 100 percent rpm, he explained, then has an engine failure and lowers the collective and pulls aft cyclic and loads up the rotor system, the rotor blades could overspeed. The AStar, he said, “will increase rpm quickly or lose it quickly. It can be a very temperamental autorotation.”
But when it comes to cyclic back during autorotations, he concluded, “I don’t know of anybody who doesn’t teach it; it’s basic aerodynamics. If [an instructor is] not teaching it, he’s setting somebody up for failure.”