From another thread.
How did you get into a flat spin?
Early morning air combat mission. 2 v 2 vs. F-16Ns. Tough fight. My wingman was defensive against one of the F-16s. I was neutral. The only way to win this thing was for me to shoot the F-16 on his tail (which would give an advantage to the one I was fighting) so that he could shoot the one with which I was engaged.
So, I went from a full AB, high G turn to the right, rolled, pulled, and turned left at high AOA, slowing down dramatically to line up the sidewinder shot. I sacrificed all of my airspeed for the shot. A calculated risk. But the stick was full aft and left. High AOA, well above critical, and full roll input.
At that AOA, with a full roll input, the airplane departed controlled flight. Much like Nasty’s departure. I was pushing it, hard, for a shot, and it didn’t like it.
It snapped off to the right (even though the stick was to the left), and the spin arrow came up on the RIO’s display (with greater than 30 degrees/second yaw rate, it is displayed and weapon system information is blanked out). Airspeed was zero. Attitude was about 10 degrees nose low and the wings were level.
Flat spin.
I went stick forward, neutral lateral (see the photo of the NATOPS manual) full left rudder, instantly, and locked my harness. I verbalized those actions on the ICS. The spin arrow came up, and the airplane yaw rate was increasing.
I went stick full into the spin (see the photo) and verbalized that. We departed at about 15,000 feet. We were descending 1,000 feet every two seconds. 10,000 was the ejection altitude (that provides margin for altimeter lag and seat separation). The spin wrapped up, and within a turn or two, we exceeded 150 degrees/second on the spin scale. I was pinned against the harness by about 5 G, perhaps more throwing me toward the instrument panel, but because I had locked the harness, I could still move the controls.
Two more turns at 150+ degrees/second.
I said (and did), “I’m going Roll SAS on and stick full into and aft”. The Roll SAS switch was small, mounted on a panel aft of the throttles, along with several other small switches for the AFDS. I hit it forward. Not easy to find. Not under extreme G with a wildly spinning airplane. It was already forward*.
With SAS on and the stick full into and aft (vs forward as required by the initial part of the departure procedure), the yaw began to decrease, along with some airframe buffet. I watched the spin arrow and displayed rate begin to decrease, from 150+ to 120, then 90, then 60, and then it went blank, meaning less than 30. The forward G was subsiding.
I went full forward on the stick, neutral lateral and the nose dropped. I neutralized the rudders.
The jet was responding to pitch controls and the AOA was now under control. Luckily, both engines kept running. That’s when I said, “I’ve got it. I’ve got it - stay with me” because we were passing 10,000 feet.
Our airspeed was about zero. But even with zero airspeed, the airplane was flying, it was responding to flight controls.
We called “knock it off, knock it off” to end the engagement (that’s the safety call) and finally pulled out about 7,000 feet at 250 knots.
I came out of AB, we rendezvoused and went home.
When we debriefed the mission on the TACTS Range (a fully instrumented 3 dimensional recreation of everything we did, enabled by a pod that we carried on a sidewinder station, that relayed flight parameters, precise position and weapon system inputs), the Range Safety Officer, a RIO named “Cracker” McCracken asked me if I could stay afterwards to answer a question.
So, I stayed. He said - “Astro, I saw this - zero airspeed, the airplane not moving over the ground and this crazy yaw rate” and he played the recording. Yep, the instrumented pod caught it all. 90 degrees AOA. Zero airspeed. yaw rate 180 degrees/second.
“Astro - did you guys get into a flat spin?”
“Yeah, Cracker, we did. It was a wild ride.”
“That’s a mandatory eject! How did you get out?”
“I went ROLL SAS on and stick full into and aft”
“I’ve never seen anyone recover from a flat spin”
“My guess is that the increased differential tail (stabilator) authority from ROLL SAS on gave us enough to slow the yaw rate. It’s hard to find that switch under G, but it made a difference.”
“D***”
*ROLL SAS was already on when I went to move it forward in the spin. This is a critical point - because it was supposed to be off. Pre-combat checks include ROLL SAS - OFF.
That’s because ROLL SAS increases your chance of departure.
A stability augmentation system moves the flight controls to steady the airplane in that axis. The yaw stability, sometimes call the yaw damper, is common on every swept wing jet aircraft. The F-14 has stability for pitch, roll, and, yaw, and each of them were controlled by separate switches.
With the ROLL SAS on, the flight control system tries to steady the airplane in roll.
But at high AOA, things get a bit weird. As AOA increases, lift increases, and drag increases. As you go past critical AOA to high AOA, drag continues to rise, but lift decreases.
So, you make a roll input. That increases the AOA on the wing you want to go “up”, which, at very high AOA, causes a loss of lift.
This is much less true in an airliner, because of something called “aerodynamic twist“. At high angle of attack, the root section of an airliners swing will stall, but because the wing is slightly twisted along its length, the outboard portion of the wing is actually still at a relatively low angle of attack, so you have good aileron authority.
But in a fighter, especially the F-14 which had unique aerodynamics in many ways, things are different. So, in the F-14, at slow speed, roll control is provided by spoilers on the down going wing, as well as differential horizontal tail input. The stabilizers, or differential tails, or stabilators, move independently for roll control.
Remember, I said things get weird at high AOA. So, you want to roll left, and put in left stick. At high AOA, the left spoilers deflect and the left stab goes down and the right stab goes up. But, at very high AOA, greater than 25 degrees, because of all that, the airplane feels the drag increase from the lift created and it yaws to the right.
You roll left, it yaws right. The opposite direction.
If you keep trying to roll left, the airplane will actually roll to the right because the right yaw creates more lift on the left wing. This is a combination of two phenomena known as both adverse yaw, which is yaw caused by roll, and proverse roll, which is roll created by a yaw.
So, you roll left, the airplane goes right.
I have actually used this when maneuvering the airplane at very slow speed. In a slow fight, at very high AOA vs. an F/A-18, I wanted to go left to maneuver for guns, so, wanting to go left, I put in a good amount of right stick, and a touch of left rudder. The airplane rolled left just as I wanted.
Back to the effect of the stability control. It knows that you are trying to roll left, because it sees the stick position, and it sees that you are actually going to the right, so it increases the amount of left control input, and makes the whole problem worse. The harder you try to roll at very high angle of attack, the more the airplane goes to the right, and the role stability. Augmentation doesn’t understand that and put more left roll.
All this stuff was fixed when we went to a full digital flight control system, but the digital flight control system did not allow the full amount of differential tail movement that the old system did.
My mistake, and I blame the fact that it was a 0500 brief, for a 7 AM launch, was to leave the roll stab on when it should’ve been off.
This stab on caused the airplane to be more prone to departure, as described above. When I slammed it over to the left, and the airplane hesitated, the roll stab put in more left roll, and we were off to the right in a departure, that wrapped up in a second to become a flat spin.
