F-14A Tomcat Uncontrolled Flat Spin Incident Investigation (Paper)

[AVeryNiceSpacePenguin]

                                                           F-14A Tomcat Uncontrolled Flat Spin Incident Investigation

                                           An investigation on the 1987 flat spin incident in the Gulf of Kerbin

 

Published day 14, year 1988 in Kerbal Aerospace Journal

 

 

       After the incident over the Kerbin gulf in 1987, when a F-14A Tomcat flat spun on approach to KSS Lexington, Grumman engineers began a test to find the cause of such a phenomena. The research data from the test has proved useful, however, the cause of the flat spin still remains unknown. This study will attempt to find probable causes to the 1987 incident, as well as 2 more incidents involving flat spin.

Case/Incident

      On day 127, year 1987, at 2hrs 56min local time, an F-14A Tomcat from VFA-127 was on approach to KSS Lexington, when around 75m/s, the radar intercept officer noted strange turbulence around the aircraft. It's current heading at this point is 056 degrees NEN North East North, and is turning to approach KSS Lexington, whose runway deck was at 76 Degrees NE North East, with wind speeds at a reasonable 5m/s. The pilot was applying yaw force to turn the aircraft when it inexplicably entered a flat spin, at ~275m ASL. The pilots attempted to recover, but were unable to, and both had to eject. Both pilots splashed down safely in the sea, while the aircraft crashed 1 km away from both pilots [Fig 1]. Black box data shows that the aircraft was dangerously close to its stall speed, but this was augmented by the flaps being fully deployed.

 

https://imgur.com/29zdGBP

[Fig 1]

 

 

Flight Logs:

T-0:00  Liftoff

T-1:14 F-14A begins approach to test area

T-2:12  F-14A Piloted by Jebediah Kerman as Pilot and Bob Kerman as Test Engineer begins its deceleration to near stall speed ~70m/s

T-2:14 The plane begins artificial spin through rudder

T-2:20 Jebediah Kerman lets go of all controlls

T-2:23 Bob Kerman notices that the aircraft has not entered into a flat spin

T-2:35 Jebediah Kerman levels off the Tomcat, and begins ascension to 3000M

T-3:35 Jebediah Kerman lifts the aircraft up almost vertical while applying adverse yaw

T-3:50 The F-14 begins descention form 4500m

T-3:50 Engine 2 flameout

T-3:50 - T-4:15 Descent from 4500m to 1500m

T-3:55 Engine 2 re-ignition

T-4:20 Jebediah Kerman levels off the Tomcat, after being unable to perform a flat spin after 2 tries

T-4:50 Jebediah Kerman begins final ascension to 7000m

T-5:50 Climb rate reduced from 50m/s to 35m/s, engine throttle from 75% to 35%

T-6:20 Final altitude achieved, speed of aircraft reduced from Mach 1.1 to subsonic Mach 0.75. During trans-sonic transition, both pilots report increased turbulence, as yaw input is added

T-6:35 Bob Kerman lowers flaps 100%

T-6:45 Deceleration achieved: airspeed now Mach 0.2 (stall speed)

T-6:46 Flight Engineer Bob Kerman introduces extra authority to the rudders

T-6:48 Jebediah Kerman announces that his air speed is 72 m/s. Actual airspeed 67 m/s; likely pitot tube malfunction

T-6:48 - 7:12 3rd flat spin attempt failed

T-7:13 Jebediah Kerman levels the F-14 at 175m

T-7:19 Jebediah Kerman radios to return to base

T-8:20 - T-12:11 Transit to runway and landing

Figures:

Test Pilot Jebediah Kerman puts the F-14A into a vertical climb, to test the effect of AoA on flat spin entry

 

The chase plane follows closely and carefully as the F-14 begins its 3rd and final spin entry attempt

 

Data

 

     The resulting data from the test flight has confirmed 2 things concerning the incident; 1.) the deployment of flaps does not affect whether or not a plane would go into a flat spin. Moreover, the deployment of flaps would 2.) improve low speed characteristics of the aircraft. Furthermore, the test proved that forcing an aircraft of the likes of an F-14 Tomcat into a flat spin is difficult or near impossible, partly due to the fact that the cause of the incident is still yet to be fully understood. It is to be noted that the environment that the testing took place in was different from the 1987 incident, however, the following incidents 41 days and 153 days from the original incident were different too, being that the latter two were in much cooler territory than at the KSC or the Gulf of Kerbin.

 

Main Investigation

      The data from the test is inconclusive on the exact parameters required to force a flat spin, however, the data can still be used for investigation on this aerodynamic phenomena. Among the data is the conformation of the positive effect of flaps during low speed flight, which was originally presumed to be a factor for entering flat spins. Comparing flight data from the test flight and the 1987 incident shows that both air craft likely had similar wing loading (both aircraft were at 25% fuel), as well as airspeed and angle of attack before the spin. However, the altitude at which the test plane began its spin entry attempt was over 40 times as high as the original incident. This discrepancy means that the stall speeds are different when compared from the ground. When measured to the airspeed indicator, stall speeds at either altitudes appear to be the same. Despite the similar conditions, aside from altitude, a true flat spin, not augmented by yawing or any input, was not achieved.

        There is a possibility that yawing has nothing to do with entering a flat spin, or isn't essential to entering a flat spin. Instead, it could be possible that the adverse yawing associated with flat spinning could be due to the fan blades 'backfiring', spinning counter to its original rotation, causing a large amount of air to be ejected both ways. However, during the test, when the both engines were shut off, or at low power, there is little to no chance of the blades of the compressor or turbine spinning opposite of its intended power. Moreover, pressure reading from the engines in all incidents and the test flight show that there was no 'backfire'. Ironic, given Jebediah's ability to love up any airplane he flies, it would seem simply backfiring an engine would be simple as 123...

Conclusion

       While there is still very little known about what could trigger a uncontrolled aerodynamic flat spin, research from the first test flight has proved essential in debunking or proving multiple hypotheses brought up concerning this aerodynamic phenomenon. Given enough test flights, it is very possible that the cause of these incidents could be found and possibly should be enough test flights to keep Jebediah fed and full,