The Trouble with Twist Grips

The Trouble with Twist Grips

By CDR “Dangerous Dave” Diamond, USNR (Ret.)

As the Navy transitions from the TH-57 Sea Ranger to the TH-73A Thrasher, one common danger remains with the new aircraft: both of these airframes use twist grip throttle controls. At first glance this shouldn’t be a big issue, but based on years of experience with students making errors with the twist grip I would like to explain why these can and do occur.

Most students are exposed to twist grip style throttles from multiple platforms prior to arriving at flight school. This can be on motorcycles, all terrain vehicles, jet skis, or even outboard boat motors. While these twist grips are fairly common, they work opposite in a couple of ways than a helicopter twist grip.

The majority of non-helicopter twist grips are twisted toward the user to increase power. A helicopter twist grip is twisted away from the user to increase power. Also, the majority of non-helicopter twist grips are manipulated with the right hand, whereas helicopter twist grips are manipulated with the left hand.

So, previous experience with twist grips could actually be negative training for a helicopter twist grip. And, in a stressful situation, this negative initial training can be what the student defaults to using.

While serving as an instructor in HT-18, I saw numerous times a student would roll off power rather than increase power. It was usually during a maneuver when twist grip was a bold face item. When they felt resistance turning it away from their body, they became confused and rolled it towards the idle position. This could make things pretty interesting for me as I attempted to avoid terrain and over-torquing the aircraft simultaneously.

When I taught new instructors in the HITU, I would do this as a common student error and it usually caught the instructors under training by complete surprise. I had one new instructor tell me he was so glad I demonstrated that because a student did it to him during his first few weeks as an instructor.

Another thing students do when stressed is what I like to call going into the fetal position. When they become overloaded, they don’t see or hear as well, and as they draw within themselves, they will sometimes inadvertently roll the twist grip towards idle.

Also, the mundane task of starting the aircraft can cause problems if the student makes twist grip errors. I have had students overtorque and overtemp aircraft during the start sequence. During the overtorque, instead of moving the twist grip to idle on start, the student rolled it to full open. The engine fuel control scheduled fuel normally and sounded normal until a rapid acceleration that actually caused the aircraft to yaw and placed the plane captain in danger. The overtemp was caused during a hung start where I directed the student to abort the start. Instead of turning the twist grip off, the student turned off the starter switch. All of the fuel pooled in the combustion chamber ignited and no air was being moved through the chamber to cool it. I immediately turned on the starter switch and rolled off the twist grip, but saw the TOT gauge hit 900 degrees Celsius. We were lucky, the starter cooled it back down quickly and no damage was noted during the inspection.

If you get orders as an instructor, remember training devices will break. My CO told me when I checked in, “It’s Driver’s Ed, before you leave here, you will probably overtorque, overtemp, and have a hard landing with a student.” I managed to have two out of three and both involved the twist grip when starting the aircraft. Do your best to keep an eye on what the student does with the twist grip. Lightly monitor with finger tip pressure the twist grip to feel if it is rotating and check the stripe when necessary to make sure it is full open.

I used to ask my onwings prior to their first flights if they rode motorcycles, all terrain vehicles, or jet skis. I then told them they were banned from using any of those devices until they completed their solos.

Also, never forget, the Flight Training Instruction lists “Common Student Errors." You never know when your student is an innovator and will find new ways to make errors. If you have an innovator, let other instructors know. It may save their lives.

About the Author

CDR David D. Diamond (NHA Lifetime Member #367) flew the SH-2F Seasprite (LAMPS MKI) with HSL-30 Det ALFA, Neptune’s Horsemen, and HSL-34, the Greencheckers. He was a Selected Reservist assigned to CTW-5 NR Det 282, the Elks, and provided direct support to HT-18, the Vigilant Eagles. He retired with over 3,600 Mishap Free hours of flight time. He also holds the unique distinction of wearing five different ranks and holding three different designators while assigned to HT-18.