Reacting to Low Rotor RPM

Yeah, I know RPM = life, but think about it, guys!

I did my monthly perusal of the NTSB helicopter accident reports this morning and this one jumped out at me. It’s another instance of a pilot reacting badly to a low rotor RPM situation. (You can read my favorite example of a poor response to low rotor RPM here.)

The report is short and, for some reason, cut off before the end. (NTSB seems to be having trouble with its database lately.) Here’s the story:

The commercial helicopter pilot reported that he was on a Title 14, CFR Part 91 business flight transporting one passenger and seven dogs to a remote camp. He said as he approached the camp, which was at 3,800 feet msl on a snow-covered glacier, flat light conditions made it difficult to discern topographical features on the glacier, so he elected to land at an alternate landing site at 3,200 feet msl to wait for conditions to improve. During the approach to the alternate site, just before touchdown, the pilot said the low rotor annunciator horn sounded, and he lowered the collective to regain rotor rpm. The pilot said he was unable to initiate a go-around, so he brought the helicopter to a hover, but due to the prevailing flat light he was unable to discern his height above the site, and he unintentionally allowed the helicopter’s left skid to touchdown on the uneven, snow-covered terrain. He said that there was an “instantaneous dynamic rollover” as the helicopter rolled to the left, the main rotor blades struck the snow, and the helicopter came to rest inverted. The helicopter sustained substantial damage to the fuselage, tail boom and main rotor drive system.

Glacier Crash with Dogs

Coast Guard photo of crash site. We’re taught to keep the shiny side up. Note dogs.

The two human occupants were not injured. They don’t say anything about the dogs, but since they were likely crammed into the back seat area, they probably cushioned each other and are okay, too. (Seriously, who puts seven dogs in the back seat area of a 4-seat helicopter?)

About Low Rotor RPM

Helicopter pilots have a saying: RPM = life. It means that if your blades are spinning fast enough, you should be able to fly. But if you lose RPM, there’s a chance that you might drop out of the sky (yes, like a brick) and have a very ugly encounter with the ground. Why? Because the spinning of the rotor blades is what gives a helicopter lift. If they stop spinning, they’re not generating lift. If they’re not spinning fast enough, they’re not generating enough lift to keep the helicopter airborne.

Helicopters have low rotor RPM warning systems. In an R44, it consists of a light on the instrument panel and a “horn.” The sound of the horn is very annoying and impossible to miss. (See for yourself here.) Because RPM is so important, the full system — light and horn — are required for flight.

On a Robinson helicopter, the low rotor RPM warning system kicks in at 97% RPM. Since the helicopter is operating at 102% RPM, that’s just 5 units below normal operation. But as they teach in the Robinson Safety Course, the helicopter should be able to fly with RPM of 80% + 1% per 1,000 feet of density altitude. Using this accident altitude as an example and assuming that it wasn’t above the standard day temperature of 15°C at the “snow-covered glacier” landing zone, the helicopter should have been capable of flight with rotor RPM as low as 84% or 85% (see density altitude chart). I use the word should (and italicize it for emphasis) because this is a rule of thumb. I do not recommend flying a helicopter below normal operating RPM. This rule of thumb just helps pilots understand how critical a low rotor RPM situation might be.

What causes low rotor RPM? Engine malfunction is one cause. A bad magneto or stuck value could rob the engine of horsepower, thus preventing it from keeping the rotor RPM where it needs to be. Performance needs beyond what the engine can produce is another. For example, it takes more power to hover than to fly; attempting to hover with a heavy load at high density altitude could result in a loss of rotor RPM. That may have been the situation here; the pilot was in an R44 Raven I (or possibly an Astro) at more than 3,000 feet density altitude at or near max gross weight*.

Emergency!

During flight training, helicopter pilots are trained to react to low-rotor RPM situations. In fact, Robinson helicopter pilots get extra training every two years (per SFAR 73) because of the unusually high number of low RPM accidents in early Robinson helicopters. Although modern-day Robinsons have correlators and governors to help the pilot maintain proper RPM, this special training and endorsement is still required.

Low rotor RPM is treated as an “emergency.” That means it has an emergency procedure associated with it. Helicopter pilots are drilled on the procedure until it becomes second nature.

Page 3-10 (in the “Emergency Procedures” section of the R44 Raven II Pilot’s Operating Handbook) states:

A horn and an illuminated caution light indicate that rotor RPM may be below safe limits. To restore RPM, immediately roll throttle on, lower collective and, in forward flight, apply aft cyclic.

As a result, when we hear that low rotor RPM warning horn — which is directly related to the deterioration of life-giving RPM — we react quickly to recover lost RPM. That means increasing throttle (to add engine power) and lowering the collective (to reduce drag caused by the rotor blades). Pulling back on the cyclic, when moving forward, can also help recover lost RPM by transferring energy in the forward speed to rotor RPM (which is why RPM increases during a cyclic flare in an autorotation).

Lots of folks argue about which is more important and whether you need to do all three. I think rolling on the throttle is most important but will acknowledge that it doesn’t always resolve the problem. Lowering the collective usually helps.

Hold that Collective

While that is all well and good, I’d like to make this radical suggestion: is lowering the collective to recover RPM a good idea when you’re within 10 feet of the ground?

In this accident, the pilot heard the low rotor RPM horn “just before touchdown.” I’ll be generous and take that to mean anywhere within 10 feet of the ground. So he’s coming in for a landing. He’s already got his descent going and he’s either increasing power to bring it into a hover or he’s past that point and is reducing power gently to touch down. Either way, lowering the collective will cause him to descend faster than he already is. (It doesn’t say anything about rolling on the throttle; did he?) He’s less than 10 feet from the ground. The report goes on to state that he was able to bring it into a hover but was apparently lower than he thought (perhaps because of his collective work?) and touched a skid to the snow, causing dynamic rollover.

Low Rotor RPM Might Not Always be an Emergency

I’d like to argue that low rotor RPM is not an emergency situation when you’re very close to the ground.

After all, what’s the worst that can happen? At less than 10 feet, you don’t need the RPM to keep you alive. Even if the RPM dropped to 0 when you were only a few feet off the ground, you’re not going to die. You’ll drop like a brick — a few feet. Spread the skids a little. I don’t even think the belly would touch the ground. If it did, the seats would collapse as designed and (literally) save your ass. We’re talking less than 10 feet here.

Instead of dealing with low rotor RPM when you’re less than 10 feet from the ground, doesn’t it make sense to ignore the horn and just land?

Remember, in an R22 or R44, the horn sounds at 97% RPM. In this example, he could still remain in flight with the RPM all the way down to 85%.

Let’s review. The pilot is at a critical moment of flight: landing, just before touchdown. The low rotor horn goes off, zapping his concentration. Instead of completing the maneuver he was almost done with, a maneuver that would put him safely on the ground, he switches gears to handle the sudden “emergency.” That reaction just puts him closer to the ground with him focused more on the RPM situation than the ground he could very well make contact with. As a result, he botches the landing, possibly distracted by a non-emergency “emergency.”

A side note here: I have witnessed a low-time pilot literally freezing up when the low rotor RPM horn sounded on landing. I was sitting beside him, horn blaring, in a hover 3 feet off the ground at a very high density altitude airport — 10,000 feet. His brain simply shut off. I repeated the words, “Just set it down,” three times before he snapped out of it. There was no danger, but the damn horn sent him into some kind of mental seizure. If that happens in cruise flight, he’s definitely dead.

The horn is scary. We’re trained to react to it. But is low rotor RPM recovery always the answer? I’ll argue that any time you’re very close to the ground, you need to think about the consequences of all of your actions before making them. Don’t react to an emergency that doesn’t exist.


* Doing the math… I don’t have the details of the accident flight’s weight. But if you figure two grown men weighing at least 180 pounds each and seven 50-pound sled dogs in the back, you have 710 pounds for just the cabin occupants. A Raven I with minimal equipment weighs at least 1440 pounds. So that brings a total of 2150 pounds. Add half tanks of fuel for another 150 pounds; that’s a total of 2300 pounds. The max gross weight of a Raven I is only 2400 pounds. So with my guesstimates, he was pretty close to max gross weight.

Now because I know the mentality of the helicopter forum participants who often lurk here (and then share their opinions about me in the forums they troll), I feel compelled to defend my calculations here instead of in the comments thread. (I don’t waste my time in the forums anymore.)

The dogs were “sled dogs,” which you’d expect since that’s the kind of dog most useful at “remote camps” in Alaska. They were later rescued by the Coast Guard, which airlifted them off Godwin Glacier after the crash. I’m estimating 50 pounds each, but they could easily be larger. Here’s a photo of them. (Frankly, I’m having trouble imagining seven 50-pound dogs crammed into the back seat area of a helicopter like mine. I’m also cringing at the thought of vacuuming all the shedded fur out.)

And yes, both the pilot and the passenger could have been Weight Watchers graduates weighing in at 140 pounds or less each. And they could have been wearing shorts and sandals. Or nothing at all. And there might have been only 10 gallons of fuel on board.

But my guesstimates are based on what I’ve experienced in reality. People are fat and like to bring unclaimed baggage, pilots like to take as much fuel as they can for missions in remote areas. It’s far more likely that the passenger was even bigger and had gear with him and the pilot had his tanks much closer to full than empty. But until the FAA releases more info — which is not likely, since there was no fatality in this accident — guesstimates are the best we can do.

3 thoughts on “Reacting to Low Rotor RPM

  1. Press Release about the crash:
    “Lebling, Ridington and the seven sled dogs were successfully hoisted and transported to awaiting emergency medical personnel in Seward. The men and dogs were reported in good condition. “

  2. This is some interesting stuff to think about Maria. I’ll keep it in mind, because I think you are absolutely right about this. Thanks!

  3. Hi Maria, I just wanted to say thank you for writing these blogs. I’m a new student at and find all the physical and aerodynamic forces explained in the PHAK and HFH to be dry and hard to remember on account they don’t explain how those things would affect me in real life. It’s really one thing to memorize and another to be able to apply it. Reading your blogs just puts everything in perspective for me and I keep getting ah-ha moments (and I don’t mean the band). I look forward to reading more. :)

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