In the last issue of Code One, I left you hanging (pardon the pun) just as we were getting into a deep stall. You will recall that I had discussed with you just what the flight control system is doing; how you can use different aerodynamics as a result of using this different approach to flight control systems; how your cues in the cockpit are different when you are flying the F-16; and how you can force the F-16 into a departure if you ignore these slightly different cues. I left you with a description of what a departure is and how it is really no big deal.
What might happen next is a deep stall. And a deep stall just might be a big deal if you are not able to recognize one and know how to correct your screw-up. What has happened is that there are certain angles of attack, or AOA, in the fifty- to sixty-degree range where the F-16 is essentially neutral as far as pitching moment is concerned (think of moment as a force). If you can somehow arrive in this fifty- to sixty-degree AOA range at zero pitch rate, the F-16 is quite content just to sit there forever. The pitching moment chart printed with this article depicts the contribution of all the forces acting on the F-16 to pitch it nose-down or nose-up. Although the computer is actually moving the surface before you engage the Manual Pitch Override, or MPO, you can think of the three lines showing what forces would result if you were holding the stick full back, full forward, or not holding it at all.
The magnitude of the numbers on the left side is unimportant for the purposes of our discussion. Just follow the two lines that show the sum of the pitch forces acting on the F-16 with the stabilator full nose-up or full nose-down. You can see that in the fifty- to sixty-degree AOA area, even though the stabilator is already commanding full nose-down, the pitching moment available to decrease the AOA is essentially zero. Remember I told you that the black box has already taken you out of the loop as soon as it saw an AOA greater than twenty-five degrees. But you, in all your cleverness, did something to the airplane to end up in the fifty- to sixty-degree range with little or no pitch rate. The flight control system is doing its best in commanding full nose-down. It just happens to be in the area where it is unable to do what it would like. It is now up to us to rectify our mistake and to do our part to recover the airplane.
Look at our pitching moment curve again and notice in the fifty- to sixty-degree area that, although we cannot command any nose-down moment, we can still command nose-up. How is that going to do us any good when we want to go nose-down? Pay close attention and I’ll show you some black magic to get around the black box.
Soooo, everything is trying to go nose-down but nothing is happening. We are going to be very clever and engage the MPO and pull nose up. From the pitching moment curve you can see that, in the fifty- to sixty-degree range, we have the ability to do just that. But why do we want to increase the AOA when everything is screaming to reduce it? If you pay strict attention to the curve, you see that we have the ability to increase the AOA above the fifty- to sixty-degree range where we were stuck. Notice also that, once we get to still higher AOA, we have some ability to move the airplane nose-down, if we now command the tail full nose-down. If we do push from here, we can generate a nose-down pitching rate. As we approach the fifty- to sixty-degree area again, the moment will once more go to zero. But the difference now is that we have a pitch rate established that will carry us right through the deep stall point and recover the airplane. Whew! Just when you thought all was lost. If you’d been paying attention you would not have gotten there in the first place. But once you did, you can see that recovery is possible from a less than optimum position.
How do we recognize a deep stall? First of all, we should know where the F-16 is susceptible to a deep stall. But to deep stall, we must first depart. In order to depart, we must be slow (slower than the recommended minimum airspeed maneuvering limits in the Dash One). Once we are slow, we must do something foolish, like abruptly pull back stick, or roll rapidly while snatching a lot of back stick, or try to help the roll rate with a lot of rudder. (The black box will try to fade the amount of rudder we can get under these conditions but we can sometimes fool it here as well.) If you’re paying a lot of attention, you’ll see a fairly obvious increase in the pitch attitude as the F-16 departs (in other words, the nose goes tip, regardless of the attitude of the airplane at the time). Depending on aircraft configuration, you may then see the nose slice left or right.
Centerline stores make this worse. Centerline stores also tend to make the deep stall more oscillatory in pitch. The F-16 is also not too happy with asymmetry. Something like an ALQ-131 pod at stations three or seven can make the airplane a potential handful at the higher angles of attack. There is then a brief period of calm because you’ve dissipated what airspeed you had going. From there, the F-16 will self-recover or go into a deep stall. If it is going to deep stall, there is a slight but nevertheless characteristic shudder as the airplane parks itself in this fifty- to sixty-degree AOA region. This shudder is good information, because the F-16 will seldom self-recover after giving you this cue.
Once you’re sure the aircraft is in a deep stall (but don’t be too quick because it will usually self-recover), it’s time to get serious about recovering. The deep stall can be extremely smooth or very oscillatory in nature (in pitch and/or in roll). The recovery technique is the same in either case. First, find the MPO switch. (A little practice beforehand when things are more calm would be an excellent idea.) Hold the switch in the override position and pull back on the stick. If you can determine where the nose is, the best time to pull is at the lowest point in the oscillation. If the deep stall is extremely stable (and they sometimes are), just hold the switch and pull. What you’re looking for is an increase in the pitch attitude. You’re also getting an increase in the AOA. But since the gauge is pegged, you’ll be unable to see an increase in the AOA on the instruments. Depending on the configuration, you may or may not see an obvious increase in pitch attitude. The AOA is increasing, however, while you’re holding back pressure, which is what we’re really after. If you can detect an increase in pitch attitude, then push when the nose is at its highest point. If for whatever reason you’re not sure if you can see any increase in pitch attitude, then you can almost do the recovery by the numbers. (But the idea is not to be Joe Cool by reaching over, engaging the MPO switch, and then trying to make the stabilator actuator white-hot by wildly pumping the stick.)
The time required for the aircraft to go through one cycle (nose-up to nose-down) is very close to three seconds. I know it’s difficult to establish just how long three seconds is while your body clock is running ten times normal rate, but try real hard. So if all else fails, then (1) find the MPO switch, (2) hold it in the override position (that is, outboard, left, and/or port), (3) pull back on the stick, (4) count three potatoes, (5) push forward on the stick, and (6) you should be flying again. But you’re stiff at low airspeeds, so all the no-nos that I’ve already pointed out about snatching on the controls still apply. Be smooooooth.
If you look at the pitch moment chart again, you can see both sides are nearly a mirror image of each other. In other words, the aircraft can do the same thing inverted as it can upright (from an aerodynamic standpoint, not a flight control standpoint). So it is also possible to get the F-16 in an inverted deep stall. If during your upright deep stall recovery you continue to hold forward pressure as the nose starts down, it is possible to pitch right over on your back and end up in an inverted deep stall. So, as the nose definitely pitches down and the AOA is off the peg, quit pushing on the stick. If you’re really smooth, you can even help your condition by adding a small amount of back pressure to keep the pitch attitude to a manageable level (something like sixty or seventy degrees nose low instead of nearly vertical, or worse).
I told you that you could pitch over on your back if you go too far with an upright recovery. It is also possible to end up in an inverted deep stall from the original departure (depending on what you were doing just before you departed). How do you tell the difference? Easy. If you can see the ground through the top of the canopy, you’re inverted. If the cockpit floor is hiding the ground, you’re upright. If the AOA gage is pegged at the big number end, you’re upright. If it’s pegged at the little number end, you’re inverted. If you have some small amount of positive g on the airplane, you’re upright. If you have some small amount of negative g, you’re inverted.
The only difference is this: Remember I told you, if you managed to get the F-16 above twenty-nine degrees AOA, the black box took you out of the loop, tried to reduce the AOA below twenty-five degrees, and countered any yaw rate. If you’re inverted, the black box does not try to counter the yaw rate. It is possible to develop a yaw rate (read spin) while you’re inverted. It’s not difficult to stop, however. Look at the ground near the horizon, or look at the turn needle to determine the direction of rotation. Then step on the opposite rudder to stop the rotation. It’s important to do this first because you can’t use the MPO effectively until you’ve stopped the rotation. Usually the F-16 will self-recover as the rotation stops. If it doesn’t, you’ll have to pitch-rock the aircraft in a manner similar to what I told you before.
From the pitching moment chart, you can see nearly an identical point on the inverted side of the chart. If you get to a negative fifty- to sixty-degree AOA, the same thing happens. The F-16 will stay there all day. The recovery is nearly the same, except in the other direction. Find the MPO switch, hold it in the override position, push full forward on the stick, look for the nose to go down (in this case, towards the sky), or wait your three potatoes, then pull on the stick. The nose should pitch toward the ground, the AOA gauge will come off the negative peg, and you’re flying. Remember, be smooth. Help the airplane. Don’t keep pulling and pitch it through to an upright deep stall.
I have implied that the airplane will always recover in one cycle. This is very nearly always the case, but there are exceptions. If the deep stall is very oscillatory and you pull out of phase (that is, the first pull starts as the nose is just starting down instead of being at its lowest point) or you do not hold the stick long enough in either direction, it is possible that it won’t recover the first time. Don’t panic. Even if you were out of phase the first time, you most likely have established your own phasing and the next cycle will recover the airplane. Just don’t rush the second cycle, and it should be a good one (remember the three potatoes). In some configurations with the small tail (or extreme aft center of gravity with the big tail), we have seen even three or more cycles required. So, don’t give up too soon. But in just about every case, a single, properly executed pitch cycle will recover from a deep stall. With a nominal two and one-half to three g’s available at 200 KCAS, the F-16 can pull out with some amazingly small altitude losses. But don’t press! If altitude becomes critical, admit your mistake and eject.
I have taken about a million words to tell you how to recover from a situation you should never get into in the first place, so pay close attention to what I told you in the first parts of this series. Learn to detect the subtle cues that the F-16 is putting out. Improve your situation awareness to know when you are slow. Smoothly (this does not necessarily mean slowly) approach the limiters when you’re at low airspeed and there is never any reason to depart or deep stall. Period. Not even if you find yourself well below the Dash One minimum maneuver airspeed limits.
One final, very important fact is being overlooked. The guys in the F-16 Combined Test Force at Edwards AFB, with the help of the TAC fighter pilots stationed there, have designed an excellent training program to show you first hand what I’ve been telling you here. All it involves is about three hours of briefing and one sortie to show you what you need to know to sort out this different airplane when you want to start maneuvering close to (or even beyond) the limits.
Deep Stalls: Furthermore
By Phil Oestricher
Director, Flight Test
Joe Bill’s comments reminded me of my first F-16 deep stall. It was extremely stable and entered via a rudder roll (not likely now, due to the rudder fader feature). After wandering around for a turn or so in a very slow, upright spin (also unlikely now), the airplane stabilized in a wings-level, fuselage-level, constant heading, MIL power descent – with no buffet, vibration, or noise. It looked just like a cross-country cruise, except for an indicated airspeed just under 100 knots and an altimeter trying to destroy itself at three seconds per 1,000 feet. I didn’t use the MPO, as we hadn’t invented it yet, but got the desired results from the flight test spin-recovery chute.
On two occasions I’ve had such an active deep stall in an F-16B that the dutch roll coupled into a pitching motion and the jet self-recovered - after thirty seconds and 10,000 feet! Bottom line? Joe Bill is right on when he says deep stalls can either be extremely smooth or very oscillatory.
One last thought on deep stall recovery: The recovery really ought to be over the first time the nose points near straight down. When it does, get off the MPO switch and do the instinctive thing. Hold the nose there for a few seconds (with whatever pitch command is appropriate) and then start a smooth recovery. Staying on the MPO switch after it’s no longer needed generally gets you into the other deep stall.
Deep Stalls: Furthermore II
By Steve Barter
Senior Experimental Test Pilot
My experience with MPO cycles in deep stalls has been somewhat different, due to the unusual nature of the testing we were doing. This unusual nature consisted of both symmetrical and asymmetrical AMRAAM/AIM-9 loadings combined with aft center of gravity and of allowing the deep stalls to develop for up to fifteen seconds before MPO engagement. In many cases, of course, we were looking for departures and deep stalls to investigate their characteristics. Specific test maneuvers were accomplished to cause departures, including rating through the limiter. Joe Bill said that we must do something foolish to depart. Well, my foolishness was that it was my job to fly these configurations!
Two specific test loadings come to mind for multiple MPO cycles. The first had AMRAAMs at stations one, two, eight, and nine with a fuel tank on the centerline. The second had AMRAAMs at the same stations with two 370-gallon fuel tanks and a centerline ECM pod. Both had the center of gravity controlled to the most aft limit. The first configuration took three properly executed MPO cycles to recover from an upright stall. The second one took four cycles. The problem was that these deep stalls were very oscillatory about all three axes, which caused the yaw rate limiter to work hard to prevent a spin. A part of the yaw rate limiter includes using the horizontal tails for roll, which reduces their authority in pitch. Therefore, it takes more cycles to generate the proper pitch rate or catch it when the yaw rate is low to cause the recovery. All of this is aggravated by centerline stores, especially the 300-gallon tank.
During some of the F-16 departure/deep-stall testing, we decided to wait a while instead of immediately beginning pitch rocking with the MPO when definitely in a deep stall. This was to see if the deep stalls became oscillatory. If so, would they self-recover? And if not, were they more difficult for the pilot to rock out of? The answers were yes, no, and definitely yes, respectively. I flew a configuration with a centerline ALQ119 pod. About fifteen seconds after the departure and eight seconds into the upright deep stall, I began pitch rocking. I would describe the deep stall as moderately oscillatory about all axes. This one took five cycles to recover.
Don’t jump to the conclusion that this is a C-model problem. It isn’t. It’s a function of allowing the motions to develop for a long time before using the MPO. The eight seconds was a very long time for me; none of you should ever wait that long before using the MPO in a deep stall. But make sure you are really in a deep stall before using the MPO. If you’ve confirmed a deep stall (check all your cues) that won’t self-recover, then use the MPO.
I’ll repeat that these were very unusual cases and configurations at grossly aft centers of gravity and we intentionally let some deep stalls go well beyond the normal recovery initiation point. Learn from this and observe your cues. If definitely stuck in a deep stall, use the MPO properly and you will recover.
There’s one other thing you probably ought to be made aware of. Since late 1981, the F-16 Combined Test Force at Edwards AFB has conducted a high-AOA training program. Because several of this magazine’s articles have been about high AOA topics, I thought you might like to hear about this program.
The purpose is to provide advanced maneuvering training to F-16 CTF pilots. Even though the F-16’s high-AOA regime can be described in words, it can’t really be mastered without some hands-on experience. This program gives that hands-on training and, we feel, makes pilots more effective and generally safer. It strives to make them familiar with maneuvers that can be accomplished (1) without departure, (2) with departure and deep stall characteristics, and (3) with recovery techniques. Specific objectives:
1. To familiarize F-16 pilots with F-16 flight qualities during high-AOA maneuvering, to include normal maneuvering, self-recovering departures (AOA excursions), and deep stalls.
2. To demonstrate and perform maneuvers that can and cannot result in a deep stall.
3. To demonstrate and practice the proper use of MPO.
The actual training has a ground phase and, of course, a flying phase. The trainee pilot flies in the front cockpit with an experienced high-AOA instructor pilot in the rear. The ground phase takes about three hours and includes:
1. A basic discussion of the F-16 flight control system, emphasizing the high-AOA features (AOA limiting yaw rate limiting, rudder fadeout, and so on).
2. F- 16 departure characteristics.
3. A study of prior flight test departures.
4. A discussion of departure reasons (center of gravity, pilot technique, aerodynamics, etc.).
5. A discussion of recovery techniques for upright and inverted deep stalls.
6. A review of several flight characteristics videotapes.
The flying phase consists of one flight in any F-16B (small or big tail) or F-16D, loaded with AIM-9s on stations one and nine. Fuel is controlled to arrive at the proper center of gravity for the maneuvers. Initially, students fly various maneuvers that will not result in an AOA excursion (above twenty-nine degrees AOA) or a deep stall. These generally are a one-g deceleration to max AOA; a one-g max-command, 360-degree roll (at max AOA); a max-command, 360-degree roll (from about 250 KCAS while at max g).
Next, students fly two maneuvers that result in AOA excursions above twenty-nine degrees. But the F-16 self-recovers: (1) from a high pitch attitude and slightly below 200 KCAS, roll 180 degrees and abruptly pull aft and hold; (2) from the same high attitude and well below 200 KCAS, roll 180 degrees and gently apply full aft stick.
Finally, upright and inverted deep stalls are demonstrated with MPO recoveries using the following maneuvers:
1. From a very high pitch attitude and a very low airspeed, roll 180 degrees and abruptly pull full aft.
2. From a high pitch attitude and a very low airspeed, abruptly roll 360 degrees with full flaperon and rudder.
3. From a high pitch attitude, hold wings level to zero airspeed.
4. From a high pitch attitude, hold wings level inverted to zero airspeed.
As you probably know, many of these maneuvers are not within normal flight manual limits, so don’t go experimenting. Even those maneuvers intended to produce deep stalls usually don’t; they just self-recover. They are presented here in a general form to give you an idea of the extremes we go to for this specific training.
As a high-AOA instructor pilot, I have observed highly experienced F-16 pilots who become confused or who used incorrect procedures during this training sortie. It happens. Even after a thorough briefing, knowledge of the flight manual, and intentionally departing the airplane. But after observing it first-hand and actually having to use the MPO several times in a controlled situation, they were able to rock out perfectly. Every pilot I’ve ever flown with on this sortie has said this training is invaluable.
