Lesson 7: Emergency Procedures and Advanced Maneuvers
We began our lessons on flight maneuvers with the basics of forward flight, turns, accelerations and decelerations, and ascents and descents. We combined these basics into moving around the airport traffic pattern, and then worked on hover operations. This information, combined with basic navigation or instrument lessons, will allow you to fly a simulator helicopter anywhere in the X-Plane environment. Now its time to talk about emergency procedures.
When an airplane loses engine power, the wings can be relied upon to provide lift as long as airspeed is maintained. The glide that can be expected can be between 10 and 60 times the altitude of the aircraft, depending on the environmental conditions, the aircraft gross weight, and the type of aircraft being flown. In a helicopter, you dont have the luxury of a good glide ratio, but all is not completely lost, and there is in fact one positive aspect. Forced landing areas for airplanes must be long enough to accommodate a reasonable landing speed and run-on landing, but helicopters can be put down in just about any clear area and still allow you to walk away. The maneuver to accomplish this is referred to as autorotation because the engine will no longer power the rotor, but rather by the air moving through it as the helicopter descends. All helicopters incorporate a clutch at the engine input to the transmission to allow the rotor to always turn faster than the engine when required. During start-up in a piston aircraft you will be required to manually engage another clutch after engine start so the engine will not be started under load, but this mechanism is never manually adjusted in flight.
The helicopter maintains three methods of storing energy that can be of use if there is a need to autorotate, and they can be used in various combinations to effect a safe completion of the maneuver. These are: rotor speed, aircraft speed, and aircraft altitude. As we discussed previously, maintaining rotor speed is essential, but we can compare the relative quantities of use of these energies in the aircraft using the example of the Robinson R-22: a drop in rotor speed from 104% to 75% is equal to a drop in airspeed from 90 to 84 kts, or 60 to 50 kts, or 32 to 0 kts, or a loss of 46 feet of altitude.
From this information you can see that losing rotor speed is your least favorable option to make use of the energy available to you because it offers the least benefit and is required to continue to fly. On the other hand, the innate energy contained in the rotor system (compared to the energy contained in the aircraft body) determines to a large extent its ability to successfully enter and complete an autorotation. The R-22 has what is referred to as a low inertia rotor system, which means that there is a relatively small amount of energy stored in it (compared to the energy contained in the aircraft and compared in a similar manner to other aircraft), so rotor rpm loss will be very rapid. This works two ways, as a loss of rotor speed can be readily regained, compared to high inertia systems where rotor speed loss will occur over a longer period of time but will be more difficult to regain. The energy stored in the rotor becomes important at the end of the autorotation when you will use it to cushion the landing after you have lost just about all altitude and airspeed.
This balance between rotor energy and aircraft energy is determined by the designer to optimize control performance (light rotors offer improved control responsiveness) and autorotative safety. These energies can be compared in what is called the autorotative index. Aircraft with a high index offer very stable autorotations, while those with a low index can be a definite E-ticket. For example, the H-53 helicopter series has a proportionately light rotor system compared to the work it does, but if this rotor could be adapted to the R-22, the aircraft would take a very long time to descend. Helicopter lore has it that the Bell 206 can autorotate, land, and then have enough rotor energy left over to pick up and move the aircraft to another location. This aircraft has a very high inertia rotor system.
Using flight test data the manufacturer will construct a height-velocity diagram (also called the deadmans curve) that will show the conditions of safe operation of the aircraft. There are two parts to the height-velocity diagram: the high altitude/low airspeed portion and the low altitude/high airspeed portion. What the curve is telling you is that if you are in a hover or low airspeed condition at low altitude (draw three points on a graph: 50 feet at 0 kts, 400 feet at 0 kts, and 200 feet at 30 kts, and connect them with a curve the area bounded by this curve will approximate a typical low-speed portion of the height-velocity diagram) you will have insufficient airspeed to successfully enter autorotation and safely land. Similarly, the area from 30 kts on up and below an altitude of about 10 feet is also dangerous because you will have too much airspeed but insufficient height to safely enter and complete an autorotation. Aircraft with more than one engine will have a much smaller low speed portion of the curve. Depending on what your conditions are with respect to the curve will determine how you should react to an engine failure. The height-velocity diagram is affected by density altitude, and with some aircraft under certain conditions, the two portions of the height-velocity diagram can meet. This results in a condition where the aircraft cannot safely depart or arrive because you must pass through a region where you cannot safely enter and recover from the autorotation.
This maneuver is performed if you lose power while in a low hover, but will be somewhat difficult to perform in the simulator because of the coordination required to cut the mixture with the mouse and control the aircraft with the sticks. Start by facing into the wind at an altitude of 3 to 10 feet, and maintain a stable hover.
In a real aircraft, a practice autorotation would be entered by cutting the throttle, but in the simulator it is essential to cut the mixture (the red sliders adjacent to the throttle control). The mixture must be cut because in the simulator the throttle is typically correlated with the collective position, so as you raise the collective you will be re-engaging the engine.
It is important to keep the engine from providing any power to the rotor. To begin the maneuver, cut the mixture with the mouse. Immediately apply right pedal to arrest the left yaw, and apply cyclic to stay over your spot. A little forward motion is acceptable, but sideward and rearward motions are to be avoided.
As the aircraft begins to settle to the ground, slowly apply up collective to cushion your landing.
Entering the Autorotation From Altitude
Begin the maneuver by cutting the mixture with the mouse. If you take no other action, two other effects will become immediately obvious: the aircraft will yaw to the left and the rotor speed will drop. Therefore your first reactions should be to lower the collective and apply right pedal.
Several years ago there was a fatal accident where an aircraft lost engine power and the pilot apparently never lowered collective. The wreckage was contained on a small roof with almost no forward motion, and witnesses described the rotors as barely turning. Ultimately the accident was traced to contaminated fuel, but poor pilot reaction to the situation resulted in the loss of 4 lives and one aircraft. Once rotor speed has fallen below 70%, recovery will most likely be impossible.
Check the trim of the aircraft by monitoring the slip-skid ball, and adjust pedal as necessary. Note the rotor speed and work to keep it in the high end of the green area by adjusting the collective. Also check cyclic control so you are flying in the direction desired and heading toward an airspeed of about 65 kts. In time, the coordination of controls on entering an autorotation should be almost simultaneous. Once you are in trim at a stable airspeed and rotor speed, you have successfully initiated the autorotation.
Unlike regular flight, higher rotor speeds may be acceptable during autorotation on some aircraft. For instance, the Enstrom F-28 allows rotor speeds of up to 120%. If collective is all the way down, the rotor speed may climb to an unacceptably high level, so you should be prepared to raise it a bit to keep the rotor speed in check. I have flown aircraft where the blades have been set so as not to require addition of collective to check rotor speed in autorotation. While this may appear to reduce workload, it also can lead to dangerous bad habits when you fly aircraft that are not rigged in this fashion. Also, acceleration of rotor speed may be slightly reduced.
It is also important to note that in a real piston powered aircraft you should add full carburetor heat whenever collective is lowered with an intent to descend. This is true for normal descents as well as practice autorotations. Adding carburetor heat when practicing autorotations is one piece of insurance that the engine will be available should you choose to terminate the maneuver. In a real aircraft, it is also wise to crosscheck the engine speed after cutting the throttle to ensure that you will only be practicing the maneuver and not performing a real autorotation. Failure to add carburetor heat at the appropriate time is one cause of engine failure that can lead to the need to perform an autorotation. Obviously you should follow the explicit instructions for application of carburetor heat for the aircraft you are flying.
Maintaining the Autorotation
Now that collective and pedal have been set, and you are attempting some semblance of control over airspeed, continuation of the autorotation should be fairly easy. Select a landing spot into the wind and maneuver the aircraft to accomplish this. You may have to turn 180 degrees. You may have to turn 180 degrees, fly past your landing point, and then turn back another 180 degrees (remember, in a real aircraft your life depends on selecting an appropriate spot, with considerations of its size and the wind direction). As you maneuver the aircraft to your spot, you should also be adjusting airspeed to slightly above the best endurance airspeed (nominally 65 kts).
For the most part, that is about it. No adjustment of pedal or collective is required, and your airspeed is also constant. The best endurance airspeed offers your least rate of descent, so you can take advantage of this to mentally prepare for the landing.
You should develop some idea of the glide capability of your aircraft in autorotation. For the R-22, best glide is about three fourths of a mile for each 1000 feet of height (about 4.5:1). The POH lists the conditions for best glide as 65 kts and 90% rotor speed.
Completing the Autorotation
As you continue descending toward your selected spot you will need to assess how close you will come to it. If you think you will be a little short (oh look I must clear those trees), you can increase airspeed to the best range speed (nominally 80 kts) and increase collective a little. You will descend a little faster, but you will gain more distance over that altitude loss. It is acceptable to take a small loss of rotor rpm here, as you can regain it by lowering collective and slowing back to 65 kts when you think your spot is made. Similarly, if you think you will overshoot (oh look I am going to land in the base of those trees at the far side of the clearing), you can momentarily slow down, but try to stay above 40 kts. You will again descend a little faster, but you will not gain as much distance. Again, restore airspeed if there is time once you are sure you will make your spot.
Now for the fun part. As you cross 40 feet height (use your radar altimeter, or in a real aircraft use the height of ground features such as trees or buildings, but try to keep your eyes outside the cockpit and looking in front of you), the ground will appear to rush up toward you. Begin to slow the aircraft to bleed off airspeed and vertical speed by applying aft cyclic. There is no need to adjust pedal, as you will not be changing power. Losing vertical speed is far more important here as you can always slide on the ground a bit. The goal is to have minimal vertical speed at about 5 feet above the ground. If you still have some forward speed here, you can pull back a bit more, but try to avoid any climb (and definitely avoid planting the tail in the ground or planting the rotor in the tail boom). Slowly move the cyclic forward so the aircraft is level, and as the aircraft begins to settle to the ground, slowly raise collective to cushion your landing.
Full autorotations to the ground are rarely performed for practice in real helicopters because of the risk of damage to the aircraft. Rather, once you have stopped forward and vertical motion, apply collective (remember, your engine should still be working) and enter a hover.
As we discussed previously, when the aircraft is in a high hover (or a steep approach or downwind landing situation) it is possible to enter a situation called power settling if the rotors start working on its own downwash. Even though you may be applying a lot of power, the aircraft will begin falling to the ground at a rapid rate, and applying more power will not help you recover. You can usually determine that power settling is occurring because there will be a large increase in vibration. To recover, lower collective and move the cyclic forward to lower the nose and fly out of it. Be prepared to lose between 500 and 2500 feet of altitude before recovery, which will be some condition of normal flight.
Ground resonance is a rare condition that can occur with a fully articulated rotor. If the landing gear touches the ground abruptly, the motion may cause the rotor blades to move out of phase with each other (remember that they can lead and lag) and unbalance the rotor head. With normal rotor speed, lift the aircraft away from contact with the ground and hover until the rotor stabilizes. With low rotor speed, lower collective to place the blades to low pitch. I dont believe X-Plane will support a demonstration of ground resonance.
Aircraft with teetering rotor systems are more prone to a condition called dynamic rollover than those with articulated systems (although my first training aircraft, with an articulated system, was lost this way). This can occur if you catch a skid on the ground and the aircraft will roll about it onto its side. The motion is extremely quick, and usually unrecoverable. You will just be unable to react fast enough. Dynamic rollover can occur during a slope landing, or even if a skid ices to the surface. The best way to get out of dynamic rollover is to just avoid it as you touch down or lift off. Check that the aircraft is free of the ground, or if skid icing is suspected, move the pedals slightly until you are sure you will break free. I dont believe X-Plane will support a demonstration of dynamic rollover.
Mast bumping is a condition that only occurs in teetering rotor systems. If you encounter a low g situation the aircraft body may be free to teeter beneath the rotor system (as the rotor system is free to teeter above the aircraft). Although the rotor is still providing lift, it cannot provide control to the aircraft, which is free to move any way it chooses. The resulting attitude of the aircraft may cause the rotor support mast to be bumped by the rotor head. This causes a local defect in the rotor mast, which then shortly fails in fatigue. The rotor head will then depart the aircraft. The way to avoid mast bumping is to keep away from low g situations such as rapidly pushing the nose over at the top of a cyclic climb. I dont believe X-Plane will support a demonstration of mast bumping, so feel free to enjoy your low g maneuvers. Just dont transfer these maneuvers to a real aircraft with a teetering rotor.