Mr Coyle is a former CF helicopter engineering test pilot who flew in 427 Sqn and AETE. A graduate of the Empire Test Pilot School, he has been an instructor at 3 schools of flight testing, and held a position at Transport Canada. He currently writes for Vertical magazine, and is the author of ‘Cyclic and Collective’ as well a being an expert witness on helicopter accidents and the Technical Marketing manager for Marinvent Corporation. He has made many presentations and teaches seminars on helicopter matters. So far, he has survived all his aviation mistakes...
It was a relatively good time to learn about using the searchlight on the Twin Huey. Overcast, big military range area with no lights, not much wind, and a reasonable proficiency at flying the machine during the daytime and IFR. I was just transitioning to the Twin, having completed the basic helicopter course and was about halfway through the Twin Huey course.
Tonight, we were going to be training on use of the Firefly searchlight system. Firefly predates the Nightsun by several generations, and was a big cluster of landing lights which could be adjusted in beam width by the operator in the back. One of our trusty (and experienced) flight engineers was the Firefly operator. James (we’ll call him) was the instructor and I was the student.
We briefed, started up, did the normal checks of the aircraft, checked the Firefly, and launched into the Stygian blackness of the Gagetown Training area. Once away from the base complex, it was dark. Very dark. Up and away, we slowed down, opened the sliding door, positioned the searchlight and started out with line searches and turning around a spot on the ground. I was learning how to cross check between flying the light and flying the instruments, and was having no difficulty. This was fun, and the light appeared to be pretty effective. Communication between student pilot and experienced engineer was going well, and I was comfortable with the way things were developing.
We came back a bit closer to the base area and set up for the next situation, which was to hover while the light was placed and kept on a spot. The target was selected from the dim glow of the reflected lights from the base, and we positioned ourselves for the maximum training benefit. In this case, maximum training benefit would have the helicopter pointing into the black nothing that was over the range area. No lights, no horizon, nothing.
I remember being aware that transitioning to hover in this condition would put us downwind, but we were light in weight, and had plenty of power in hand, so this shouldn’t be a problem. I started the transition gingerly, and there were a few vibrations, but that was normal for a downwind type approach. The light was shining on the house, and I was taking my time – not much to look at straight ahead, and I was aware one could get sucked in by just looking at the house. The instructor who was watching the target, I seem to remember thought that the transition was taking a bit too long, and he said – “Raise the nose just a bit more, and lower the collective just a bit.” Being a dutiful student, I did as requested, not by looking at the target (which was off to one side by now), but by a quick glance at the attitude indicator. I don’t recall any other indications of impending doom. There were vibrations, but nothing unusual – the target seemed to be in about the right place, power seemed to be OK, when suddenly the engineer yells– pull UP pull UP PULL UP!
The next few seconds were pretty confused. The instructor immediately yanked on the collective and then took control (in that order). We started to climb, or at least, it felt like a climb to me. I do remember the low rotor warning horn coming on and going quiet at least a couple of times. The torque was pretty high (about 110% or so) and we were going up like an elevator. At least the engineer stopped yelling and as we were within spitting distance of the heliport, we entered a tight circuit and landed. I don’t remember if we declared an emergency or not, but within a few moments we were on the ground and walking into the maintenance office.
Only 110% torque? The maintenance people said, “Hmm, tricky.” A lot of digging through maintenance manuals, and several cups of coffee later it was decided that only a visual inspection was needed if the torque was less than 113%. I’d just had my first encounter with the torque limiter on the Twin Huey. (I later discovered exactly how this piece of equipment worked, and it became a personal vendetta to make sure nothing like this work of the devil ever got installed on any helicopter I certified. For those Twins that I did post-maintenance flight checks on, I made sure it didn’t come on until at least 105%) By the way, it worked by actively reducing fuel flow any time the torque was above the torque limiter setting – so if you hit the limiter and really needed the power, you were on a very short road to no-where.
For those who are interested in what really happened, all I can surmise is that since we were downwind, and got into a bit of descent, we entered vortex ring state without knowing it. Only the quick response of the engineer saved us. We didn’t recognize it because we had no visual references and no experience of what the symptoms of vortex ring state might be.
This crew was fortunate to have more power than normally available. By regulation, aircrew shall review the torque and other operating limitations of their own aircraft type for the mission planned. Moreover, any aircraft rigging shall be done as per the technical orders applicable for the aircraft type.
Vortex Ring State is a helicopter-only condition that happens when the downwash velocity from the rotor approximately equals the rate of descent. In this condition, the two competing air masses get pretty confused and the rotor starts to eat it’s own downwash. This can very quickly develop into a rapid rate of descent, and it’s necessary to get forward airspeed to fly out of it. In many ways, it’s similar to a stall in a fixed wing airplane– the symptoms are broadly the same– low frequency airframe buffeting, relatively unresponsive controls and a sinking feeling. The recovery is almost the same- lower the nose positively and then apply power. I’ve only ever been able to demonstrate the effect by decelerating at a low power condition while downwind (at a safe altitude, of course).