In daylight, with good visual references, it’s pretty hard to get your perception divorced from the reality of what’s going on around you. In the clouds, or at night, it’s a different story. The lack of things in the real world for your eyes to use to perceive motion means we have to rely on instruments — which being of earthly construction have significant limitations on how well they work and what they can tell us.
For example, an artificial horizon (whether mechanical or electronic) reflects (or projects) the same amount of light back to us whether we’re in level flight or nose up or nose down. We have to interpret this information and compare it to what we want the attitude of the helicopter to be. If the change of pitch attitude up or down is rapid, our inner ear might help with putting the total picture together, but this can be masked by turbulence, and if the rate of pitch is slow, we may not get warned by that inner ear.
We can also use other information such as rate of change of altitude, vertical speed, change of airspeed, wind noise and so on to help us understand what’s happening to our aircraft, but if we find ourselves in an unexpected situation, adrenaline may mask any or all of these symptoms.
So, it’s easy to see why we can get fooled — and it happens to every type of aerial device from small fixed-wing airplanes to airliners, and helicopters are especially prone by virtue of the proliferation of glass (which permits more distracting visual cues to be thrown at the crew) and the ability to fly at low speeds.
The inherent instability of the helicopter doesn’t help, and the ability to lift off into very poor visual conditions with little (or no) forward airspeed rounds off the list of reasons why helicopters are perhaps the most likely flying machines to cause disorientation.
Different Aircraft, Different Problem
Consider a fixed-wing airplane taking off. It won’t lift off the ground, let alone climb until it has considerable forward speed. Assuming the airplane never gets close to the stall, there’s really only one or two ways that the aircraft can wander away from a “controlled” situation — in pitch, it can only go to a nose-high, airspeed decreasing; or nose-low, airspeed increasing situation. In roll, it can go left or right. Hardly anything would ever cause an airplane to establish an unusual attitude in yaw in forward flight. So the fixed-wing pilot is faced with a relatively small number of situations that could be classified as disorienting, and can thus pretty easily be trained to recognize and recover from those situations. Even this relatively simple set of conditions, however, has caused significant grief in the fixed-wing world. Consider the crash of the Air France Airbus A330-203 over the middle of the Atlantic in the middle of night — following loss of pilot static information — as just one example.
So the emphasis on disorientation that we’ve inherited in the helicopter world has a distinct fixed-wing bias. And since nearly all helicopter instrument flight rules (IFR) flying and night flying is at speeds above VY (minimum power speed), it’s pretty reasonable to expect that this is all we need.
Unfortunately some recent incidents have shown where things really do start to fall apart in the helicopter-only regime of flying — namely at speeds near and below VY.
Say, for example, a helicopter is on an instrument approach, with the autopilot fully coupled maintaining glideslope and track. For reasons that are not clear, the machine is permitted to slow below VY. What the crew hasn’t noticed (or realized) is that as the aircraft falls below a certain relatively slow airspeed, the yaw channel reverts to heading hold — not turn coordination — and as they try to turn (using cyclic to change angle of bank), the helicopter is suddenly maintaining heading but with an angle of bank and not turning. Confusion multiplies upon confusion, and pretty soon they are out of control.
Heading is a difficult parameter to monitor at slow speeds, because small inputs can cause relatively large heading changes, but with few noticeable cues to the pilots.
The Royal Air Force used to demonstrate this by putting student pilots in a box that was on a turntable with very smooth bearings. After the lid was closed, the student would be talking to the instructor for several minutes and at some point during this conversation, the box would be rotated, but at a very, very slow acceleration, below the threshold of perception of yaw. After several minutes of acceleration, the rate of rotation would be quite high, but unperceivable by the pilot. The box’s lid would then be quickly removed and the pilot presented with a rapid rotation, but with an inner ear quite convinced it was not rotating. The scrambled signals between the two sensors often resulted in a display of the contents of the pilot’s stomach. There are other equally compelling ways to show how our senses can be scrambled — and hence the need for training to believe your instruments instead of your senses.
Diagnosing Disorientation
For those who haven’t been disorientated in an aircraft before, you need to learn to believe the instruments — after learning to recognize the symptoms of “not so controlled” flight.
And herein lies a problem for the helicopter world. While we know and practice unusual attitude recovery in forward flight (i.e. nose high, airspeed decreasing or nose low, airspeed increasing), we seldom think about how we can get disoriented in the region between liftoff and VY.
A recent incident offshore showed this problem with nearly disastrous results.
The large, two-crew helicopter was taking off from the back of an oil support ship at night in relatively rough weather. After clearing the deck, and an initial nose down pitch to accelerate, something must have triggered the flying pilot to put in a very large aft stick input — perhaps the pattern of lights of the oil support vessel that was on his side of the helicopter, or something else we’ll never know. Regardless of the source, it was compelling enough to cause him to put in a very large aft stick input , and the helicopter suddenly pitched to about 20-degrees nose up — an attitude that would never normally be seen in any flight regime. The helicopter decelerated to zero airspeed as it climbed and then started to slide backwards and down towards the water. For some reason the non-flying pilot did and said nothing — he may have been busy with other duties or so perplexed at what was going on as to be unable to offer any good advice. At some point during this, someone recognized the very unusual nature of things and attempted a recovery action — and recovery was started. But it was such a close call that the safety watch on the oil supply vessel was ready to press the crash button when he saw the helicopter rapidly descending backwards towards the water and only when it emerged seconds later from the spray that was sent up by the rotor downwash did he start to breathe again. Some post-flight analysis of the angles used and heights above the waves convinced many that there had been some divine intervention that night.
When I was told of this incident, I wondered if no-one was teaching the instrument takeoff technique any longer. That technique had certainly saved my bacon when I encountered disorientation at low level over the water, even though that had not been the intended teaching point!
I was flying a Royal Navy Lynx, and we were supposed to be firing a missile on a test range that morning. The test range was in the Irish Sea, and that morning it was covered in mist and fog. There was also a sailboat transiting the range, and we were going to see if we could identify it for some further visit by the constabulary. The navigator was giving me headings and airspeeds to fly, and I knew we were getting quite close to the boat, even though I was flying solely on instruments. The speeds kept getting slower and slower, and the turns larger and larger. Suddenly, I saw the sail go by just below us to the left, and the navigator gave a very large turn, and… I lost it! I had no idea where we were spatially, there was no airspeed, and we were at a steady height of about 100 feet above the waves. I immediately reverted to the instrument takeoff technique of “wings level, heading constant, apply power — and then looked for rates of climb on the vertical speed indicator, followed by climb on the altimeter. When I had both of those, I lowered the nose five degrees while continuing to climb, and maintained heading. When we got to 60 knots, I raised the nose and continued to climb. And restarted my heart. Only later that day in the bar did I thank those instructors and check pilots who made me fly this procedure again and again, because back then, I considered the procedure quaint but useless — if the weather was that bad, I wouldn’t be flying, so why was an instrument takeoff something I needed to do?
So, perhaps we need to rethink our emphasis on disorientation and unusual attitudes in helicopters — the slow speed stuff could be more important than the high-speed
