How Many Gs?

The accelerometer shows nearly 3.5 positive Gs¹ and 0.4 negative Gs. What happened?

The training exercise was to fly to a local proficiency base for some copilot right seat touch-and-go landings, two left seat pilot maximum effort landings, and then back to the home station for a two-ship night vision goggle formation flight.

Start, taxi, take off, arrival, and touch-and-go landings at Dobbins Air Reserve Base, GA, were all uneventful. My first max effort landing, however, had several errors. The landing was on the centerline and in the zone at ~ 450 feet. The aircraft’s main landing gear touched down first, and the aircraft bounced ~ 5 feet. The main landing gear touched down again at ~ 550 feet, with the nose wheel touching down at ~ 600 feet. The aircraft was slowed down to taxi speed with ~ 500 feet of Veda landing zone (LZ) remaining.

The landing was faulty due to the visual illusion of the airstrip. The terrain sloped down toward the LZ, but the LZ was up-sloping. I thought I was too steep on the glide path and had tried to correct it. The vertical velocity indicator (VVI) was ~ 450-500 feet per minute on the bounced landing. The loadmaster stated that some of the emergency exit lights flashed on and then off. No lights remained on, so I proceeded with my next max effort landing. This landing was considerably better than the first one. It was on speed, centerline, ~ 300-400 VVI, ~ 250 feet in the zone, and no bounce.

I took off and headed back to Maxwell Air Force Base, GA, for the formation portion. On the climb out to altitude, the engineer noticed the accelerometer was reading ~ 3.5 Gs. After discussing the situation with the crew, we decided to continue with the mission.

During the debriefing with maintenance, we asked them to pull the digital flight data recorder (DFDR) information. The DFDR showed that the most Gs the aircraft sustained was 2.47, not the 3.5 Gs shown on the climb out. As the Dash-1 (the aircraft’s instruction manual) states, the accelerometer is designed for inflight use only and does not accurately measure G forces during landing.

1 “A ‘G’ is a measurement of force that is equal to the force of gravity pushing down on a stationary object on the earth’s surface. Gravitational force actually refers to an object’s weight (Force equals Mass times Acceleration, or F = ma).” https://www.aviastar.org/ theory/basics_of_flight/index.html