High density altitude with heavy ship.
During our ferry flight from Seattle, WA to Page, AZ, we decided to make a fuel/lunch stop at Bryce Canyon airport (BCE). Although I think we could have made it to Page with the fuel we had on board — about 1/3 tanks or 18 gallons to go 60 miles — we’d barely make our legal requirement of 20 minutes of reserve fuel for the flight. We’d also be flying direct over relatively hostile yet strikingly beautiful desert terrain. Not the kind of place you want to make an emergency landing prompted by a low-fuel light.
Bryce Canyon Airport is at 7590 feet MSL. We listened to the AWOS as we approached and learned that the temperatures were in the 80s (can’t remember exactly) and the density altitude was 9400 feet. (Whoa.) There was a 4-6 knot wind coming from the north.
Louis, a sea-level pilot, was at the controls. I figured we weighed about 2300 lbs. I knew we could hover at 6300 feet/104°F at max gross weight. Although I could have pulled out the manual to double-check the performance charts for our exact combination of weight, altitude, and temperature, I didn’t think it was necessary. After all, the 6300 feet/104°F combination equaled almost 11,000 feet density altitude (consult the chart; you can click it to see a larger view on Wikipedia). 9400 feet was well within that.
And Louis did well on approach. Although he came in a little fast at the beginning, he had a good approach speed and angle — at least by my standards — as we flew into the wind for landing direct to the ramp. He even got it into a hover where we’d park. But then the low rotor RPM horn went off. The helicopter wasn’t generating enough power to keep the blades spinning at the required RPM.
We were about three feet off the ground when this happened, so it wasn’t a big deal. I told Louis to just put it down. He was either fixated on the RPM gauge or trying hard to put it down gently, because he didn’t set it right down. He drifted backwards a few feet as we descended with the horn blaring. Finally, he put it on the ground. The rotor RPM shot up, but didn’t overspeed into redline.
I should make a few things clear here, especially for non-pilots, non-helicopter pilots, and non-Robinson pilots.
- Rotor RPM is life. If your rotors slow beyond what’s necessary for lift, the helicopter will indeed drop like a brick. That’s a very bad thing.
- The emergency procedure for low rotor rpm is to lower the collective and increase the throttle. We’re trained to do this so much that it becomes automatic. But lowering the collective isn’t always practical. The pilot needs to evaluate the entire situation — primarily height from the ground or obstacles (how close are you?) and rotor RPM (how low is it?) — before taking action. You don’t, for example, want to simply lower the collective if you’re at 95% RPM 3 feet off the ground in rough terrain at 2,000 feet density altitude. (Of course, you’re not likely to get a low rotor horn in that situation anyway.)
- Most modern helicopters have electronic governors that work with the throttle to make sure the engine delivers enough power to keep the blades spinning within an acceptable range of rotor speeds. My helicopter has such a feature. It works very well — except in high density altitude situations when the collective is raised quickly. Then it doesn’t always keep up rotor demand. In those situations, it doesn’t fail — it just doesn’t always spin the blades at the ideal 102% speed.
- On a Robinson helicopter, the low rotor RPM warning system, which consists of a loud horn and a light, kicks in at 97% RPM. That’s really high and it gives the pilot plenty of time to fix the problem before it becomes very serious.
- A Robinson helicopter can fly at 80% RPM + 1% per 1000 feet of density altitude. That means we could fly, in this situation, with 89.4% RPM. It isn’t recommended, but with a good pilot at the controls, it is theoretically possible.
The horn at landing had me concerned. After all, I was at the airport to have lunch and add fuel — both of which would add weight to the aircraft. (Okay, so lunch wouldn’t add that much weight.) If I couldn’t get it flying with what we had on board, adding fuel would only make matters worse. It was midday, after all, and we’d have to wait hours before the temperature started to drop. There was a chance we could get stuck there for a while. In that case, I wanted to know before I went to lunch so we could do something interesting in the park rather than sit around the airport terminal.
So I took the controls, bought everything back up to 102% RPM, and started raising the collective. I’ve done a lot of flying in high density altitude situations, so I know from experience that it takes a certain “touch” to avoid low rotor situations. I pulled the collective up slowly, felt the helicopter get light on its skids, and kept pulling. We were off the ground at 22 inches of manifold pressure, in a nice, steady hover. The engine sounded good, the low rotor RPM horn kept quiet. Keeping in mind that it takes more power to hover than to fly, I was satisfied that I’d be able to take off at our current weight and density altitude situation. I set it back down and we shut down.
But when I placed my fuel order, I asked for only 5 gallons. That’s 30 lbs of 100LL.
The line guy at the airport told us about how he liked watching the R22s take off from the airport. He said they only come in the late fall and early spring. They almost always do running takeoffs. I kind of wondered why they’d come at all. That helicopter, with two people on board, simply does not perform well at high density altitude.
We went to Ruby’s Inn on the free shuttle they offer from the airport. Three other folks who’d come in from Scottsdale in a small plane joined us for the ride. We had lunch in the restaurant there, ordering from the menu rather than waiting on line for the buffet. I had salad. I’m trying to lose weight and this seemed like a good time to stay on my diet.
After wandering around the huge “General Store” there, we hitched a ride back to the airport on the shuttle. I paid for my fuel, stopped in the restroom, and headed outside.
I admit that I was a little nervous about our departure from Bryce. One of the reasons for this was a recent R44 accident in Washington State that involved Louis’s old flight instructor. She’d been flying a Raven I in the Snowqualmie Pass area with three passengers on board when she’d crashed on takeoff. The NTSB report is still preliminary as I write this, but most folks are pretty certain that density altitude played a part in this fatal crash.
I’d wanted to depart into the wind, using the 6 to 8 mph breeze to help me get through effective translational lift (ETL), which occurs around 24 knot airspeed in an R44. The trouble was, the wind was blowing across the ramp area and a small jet was parked at the edge of the ramp, making a low-level obstruction there. If I hover-taxied over to the taxiway, I could takeoff downhill, but with a quartering tailwind that would not help the situation. Of course, a running takeoff — that’s where you get the helicopter light on its skids and run on the skid shoes until you’re through ETL — would be possible on the taxiway, which was smooth. In the end, I decided to pick it up into a hover and take off with a quartering headwind toward the runway and big empty space beyond its approach end. I’d have pavement under me for at least 200 feet, so I could always slide along it or abort the takeoff with a running landing if I couldn’t get enough lift to clear the fence and the road beyond it. You can see all this in the diagram; we were the red X.
I started up and we listened to the AWOS. Density altitude was now 9900 feet. (I guess it had warmed up a bit because there hadn’t been earth-shifting earthquakes while we ate.) Mike and Louis were quiet as I pulled pitch and brought the helicopter into a hover. I’m not sure if they were as surprised as I was that we didn’t get a low rotor horn. I pointed the helicopter in the direction I wanted to go, pushed the cyclic forward gently, and started my takeoff run. We varied from 3 to 8 feet off the ground before I felt the familiar vibrations of ETL. Then we were climbing nicely, well clear of the fence and the road. No horn.
I turned to the southeast toward Page and flew for a while before handing the controls back to Louis.