Just Another Spin?

Flying Safety, Nov, 2001 by Greg Davis

89 FTS

Sheppard AFB TX

In May 2000, a young captain at Laughlin AFB experienced a very different view of the T-37 and the way it flies through the air.

Picture in your mind the nose of the aircraft 50 degrees high, in 90 degrees of left bank, and the airspeed decreasing through 150 knots. The mishap aircraft (MA) was flying formation on the wing of another T-37, and just as the formation reached its apex the MA's student pilot flew a little too close for the MA instructor pilot's comfort level. Being a good IP, he took the jet and began to break out from the fingertip position. The IP's initial move was a roll away from lead, which went okay through the first part of roll, but when he initiated backstick pressure, autorotation to the right began with only a very slight buffet. After applying spin prevention inputs to the controls, the IP was able to regain control and recover the aircraft. When this autorotation occurred, it became the seventh documented case of autorotation in the T-37 in the past ten years, and was not "just another spin"!

Pilots in the T-37 community recognize that spins happen because of stall and yaw and have learned this since becoming fledgling aviators. Normally, four entries are used for spin scenarios in the T-37. There are the low and high right/left combinations, which every pilot going through undergraduate pilot training (UPT) gets to see. This is where the nose of the aircraft is brought up until the aircraft starts to buffet, and you stomp on the rudder to get the spin to begin. Another entry discussed in the T-37 Dash-1 comes from adverse yaw, which causes the aircraft to roll in the direction opposite the yaw and results in a spin. Finally, there is the gyroscopic effect of the engines, which could induce a spin to the left without the use of rudders.

Looking at the breakout mentioned earlier, the roll went to the right and the aircraft spun to the right. The rudder remained fixed at neutral. Adverse yaw? No! The aircraft did not reverse the roll to the left and enter a left spin. It went right! Why? I initially thought this happened due to a phenomenon known as inertial coupling.

Most pilots hear this term for the first time in UPT during T-38 academics. However, it happens in the A-37 (a T-37 derivative) also. Simply defined, inertial coupling occurs anytime you have acceleration rates about two axes generating an acceleration about a third axis. For this example: Roll Rate Pitch Rate = Yaw Acceleration Generating a Departure = Spin! Keep in mind that inertial effects got you to this point, and they can affect the overall spin characteristics of the aircraft. The higher the energy level (airspeed) of the aircraft, the greater the angular rates will be. The higher angular rates can combine with the inertial characteristics and give you a very different looking spin.

Knowing the similarities between the A-37 and the T-37 begged the question: Could the T-37 inertially depart controlled flight under the conditions of the breakout described above?

In April 2001, HQ AETC/DOF and 19 AF/DO sponsored a flight test program called HAVE SPIN. The United States Air Force Test Pilot School (USAFTPS) had responsibility as the test organization for the program as part of their Test Management Project curriculum requirement. TPS spent $43,000 of TPS resources, with AETC donating the IP's, the aircraft and the flying hours. The program consisted of ten test missions for 14.2 hours and 178 departure-and-spin data points in speeds ranging from 100-150 KIAS. The good news is, under the conditions tested with rudder fixed at neutral, aerodynamic effects governed the departures and not inertial effects.

The team noticed two types of departures. The first, mentioned above, dealt with adverse yaw. Adverse yaw happened in all cases when full aileron was held in the direction of the roll.

The team called the second a "rolling departure." Pilot comments described the departure as an aerodynamic stall followed by a roll and yaw in the same direction of the initial input. Similar characteristics were observed throughout the tested airspeed envelope. After 90-135 degrees of roll and full aft stick input, the aircraft would pass through approximately a second of heavy buffet, then "hesitate" in bank and yaw for approximately one to two seconds. The nose would then drop, with increasing yaw in the same direction. Full aft stick was held throughout the maneuvers and any forward stick movement to break the stall worked in every case. The departure was easily induced to the left but not to the right.

The test team came up with two recommendations:

1. AETC should add academic instruction for instructor pilots and students on both the rolling and adverse yaw departure modes and how to avoid these departures.

2. AETC should add a full lateral/full aft stick spin demonstration to the T-37 instructor pilot familiarization sortie to demonstrate both the rolling and adverse yaw spin entry without rudder deflection.

A review of the past ten years of inadvertent spins in the T-37 reveals that the nose-high recovery is another maneuver conducive to spins. Out of 241 documented inadvertent spins over the past ten years, 85 came during nose-high recoveries. How many of you teach the students to just roll and pull, or roll, plant the wings and then pull? To prevent an inadvertent spin, try teaching the students to roll, plant the lift vector and then pull. The bottom line is, if you are rolling when you stall the aircraft, you will very likely enter a spin. You do not need to be afraid of breakouts; just keep in the back of your mind the possibilities and the recovery options.