Safety Check: The Danger of Throttle Chops
As the weather warms up, it’s more important than ever to be aware of the impact chopping the throttle can have on your helicopter.
A throttle chop occurs when you suddenly and rapidly roll off the throttle. In some circumstances, throttle chops can result in the engine quitting, which can have devastating consequences. Let’s peek into the different types of engines and investigate what happens inside each when you perform a throttle chop.
Piston Helicopter with Carburetor
Chopping the throttle shifts the low pressure inside the venture into a high pressure behind the throttle valve for a few seconds. Fuel does not compress like air does, so as the high pressure dissipates, the fuel that had been forced down tends to rise into the float bowl, creating a richer mixture – similar to putting the choke on for a few seconds.
Unless every adjustment on the carburetor is set just right, this is when the engine will typically quit on you. A hot sunny day can intensify this problem even further.
Piston Helicopters with Fuel Injection
Most piston helicopters with fuel injection have the fuel divider at the top of the engine, which is one of the hottest places it could possibly be. Inside the fuel divider is a diaphragm with a needle and seat. Here, when you chop the throttle, it creates a rich mixture inside (like with the carburetor). It might only be in there for a second or two, but it’s still long enough for your engine to potentially quit. As anyone who has experienced this before will tell you, you just don’t have enough hands to pull off an air start!
Enstrom piston helicopters have been fuel injected since 1976, so naturally, they too had the potential for the engine to quit after a throttle chop. But then a decade later, around the time of the “F” model’s introduction,
Enstrom engineers reached out to dealers like me for ideas to address this very issue.
They went on to develop a correlator design that works great in flight. All you have to do as a pilot is push the collective down, no finesse required. You also don’t have to worry about the overspeed of the engine, with the engine RPMs hovering just below the green. So, even in a power recovery that goes awry, the freewheeling clutch is at no risk of damage.
Turbine Helicopter with RR Series 2 Turbine Engines
This engine type was the standard for most light turbine helicopters before FADEC added digital control options. Engineers at General Motors figured out a clever failsafe so that, if the throttle linkage ever came off mid-flight, a mousetrap-like device would snap open, keeping the throttle in the 100% position. Most other helicopters at the time might have returned the throttle to idle, and if they were at flight idle on the ground, they would shut down.
If anyone hasn’t had the opportunity to handle a fuel-control helicopter, I’d highly recommend you visit a hands-on training center. Here’s why: the idle release is on the pilot side of the collective, so if the co-pilot snaps off the throttle, any slop in the system can cause a momentary shutoff of fuel. At this point, you’d better hope the machine has an auto relight feature or else you’re in for an exciting ride. Also, if the throttle isn’t retarded, you can expect an N2 overspeed.
When Enstrom designed the 480, they studied helicopters in autorotation, noting the number of helicopters that had appeared problem-free before the throttle chop, but then after, were either damaged or had an N2 overspeed without a retarded throttle. Using these observations, the engineers came up with an ingenious idea that, to my knowledge, is still unique to Enstrom models for light turbine helicopters.
Under this system, when the throttle is fully on (fly), the N2 will be 5% under flying speed. If for some reason it is not, you can adjust the N2 trim (beeper) so that it is. Then, as you begin to pull pitch, the drop compensator will bring the N2 up to flying speed well before you leave the ground. This makes the helicopter a dream for novice pilots still learning the ropes.
The speed at which you follow this procedure is a non-factor, and there is no throttle work required if you are doing a power recovery. Just raise the collective, and you are in good shape.
If you are performing a touchdown auto, then, assuming everything in the flare is set right, you can just roll the throttle off. In 2013, Enstrom designed a dual start system for the 480B, which is not only great for training student pilots on how to start a turbine, but it also solves the slop in the rigging problem because the fuel shutoff is now part of the system.
Roger Sharkey is the owner and operator of Sharkey’s Helicopters in Lebanon, New Hampshire, the longest serving Enstrom dealer and service center in the United States.
About Enstrom Helicopter
From Rudy Enstrom’s early designs in 1943 to initial testing in a Michigan Quarry in 1957 to aircraft operating on six continents, Enstrom Helicopter Corporation has maintained a reputation for safety, value and performance. Based in Menominee, Michigan and proudly made in the United States, Enstrom has a rich history for design innovation. The goal is to provide helicopters to the customer’s exact specification and deliver support and maintenance worldwide.