Forum: F-16 Procedures

"Some things are easy, like engine failure or in my case seeing the right LEF up at about 60 degrees"

Yeah man, happens to me all the time...

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VarkVet wrote:

Another “trap post”

Blaze gas and orbit for a few, then decide not to use the blow down bottle as the mains dropped by gravity, and I would lose NWS if I used the bottle. Then it hit me, GRAVITY!!!

Bottles and NWS is an F-15 thing?
You got at least 3+ attempts to unlock the gear with full bottles!
still have some braking. Shrug

I owe Gums an apology!
It is a fact that if you blow the gear down you lose NWS, even if B system pressure is still available! (Isolation Valve)
I was definitely talking out of my a$$ on that post. Embarassed

Sorry Gums. Not Worthy

(also E-braking and blow down is seperate bottles)

Roscoe wrote:

A story I heard from a co-worker back in 96...and it was old then, so no telling when this actually happened and I'm sure to goober it up but here goes

Guy was flying in AZ as a low time guy when suddenly the airplane wanted to pitch...hard. First time it caught him by surprise and before he knew what was happening he was over the top in an uncommanded loop. Since he was flying IFR home from the MOA he was promptly yelled at by the FAA controller as well as lead. As he was bottomed out of his loop he had the presence to roll the jet and turn it into a level turn. Think think think...for what ever reason he hit the MPO switch. It's designed to take a large chunk of the flight control system out of the loop but according to GD it shouldn't have worked but it did and he had pitch authority again. As he got home and tried to land he found he needed three hands. Stick/throttle/MPO. Think think think...he pulled his black USAF issue pen from his pocket, unscrewed the cap with his teeth and was able to jam it in such a way to hold the MPO switch in the engaged position. Landed uneventfully.

He got all kinds of awards for that one...to this day LM insists that the MPO switch shouldn't have worked, but then the uncommanded pitch was never explained as well so...

G-suit hose inadvertently hitting the pitch trim wheel? Shrug

salute!

Exactly my thots Vark.

The pitch trim wheel could command about 3.5 gees if rotated fully back. So I would show student studly the prettiest loop you ever saw using it. Start at 400 or even slower and run trim full aft. Then hands off stick. Nice pull and the sucker unloads at the top to stay within limiter, then increases gee on back side until you come out. Was neat.

Still don't unnerstan why the MPO helped, but i ain't arguing. What works works!

Gums sends...

Pitch trim??? I don't think so. You can override the max pitch trim with stick force. This guy's forward stick input had no effect. The brains at GD at the time, especially Scotty, spent many hours trying to figure this one out; and they had the FLCS memory data and an intact aircraft to work with.

Roscoe wrote:

A story I heard from a co-worker back in 96...and it was old then, so no telling when this actually happened and I'm sure to goober it up but here goes

Guy was flying in AZ as a low time guy when suddenly the airplane wanted to pitch...hard. First time it caught him by surprise and before he knew what was happening he was over the top in an uncommanded loop. Since he was flying IFR home from the MOA he was promptly yelled at by the FAA controller as well as lead. As he was bottomed out of his loop he had the presence to roll the jet and turn it into a level turn. Think think think...for what ever reason he hit the MPO switch. It's designed to take a large chunk of the flight control system out of the loop but according to GD it shouldn't have worked but it did and he had pitch authority again. As he got home and tried to land he found he needed three hands. Stick/throttle/MPO. Think think think...he pulled his black USAF issue pen from his pocket, unscrewed the cap with his teeth and was able to jam it in such a way to hold the MPO switch in the engaged position. Landed uneventfully.

He got all kinds of awards for that one...to this day LM insists that the MPO switch shouldn't have worked, but then the uncommanded pitch was never explained as well so...

Would love to know what block (analog? digital?) and what they did to the jet before it flew again. I would also love to know exactly what data Scotty had to look at.
But, assuming it was a fairly early digital model, I think I have a possible explanation. (If somebody could tell me that they replaced the MPO switch before flying again, I would love it). But anyway, here goes....

Within the control laws ("CLAW") there are a couple of initialization-type flags that can get set - "Basic Initialization (BI)" and "Special Initialization (SI)". Without getting into all the gory details, these flags tell the CLAW to perform a partial reset, or re-initialization, within itself. Certain filters, the pitch integrator, etc get reset, and other special filters get triggered. The primary goal of these resets is to smooth the transition between the major modes of the CLAW. When the gear handle comes down, for instance, and the CLAW switches from UA to PA modes - a Basic Initialization is triggered to "smooth out" the change.
The list of events that trigger these flags has changed through the years, but for some time (Block 40 time-frame for sure) one of the triggers for "BI" was engagement of the MPO switch. Looking back, this seems like a bit of a poor choice, as the MPO switch itself has no effect on the CLAW unless AOA is above 29. ("BI" should have really been set on the first frame where both MPO and AOA>29 were set. It is done differently now, in the most recent blocks, and the MPO switch alone no longer is in the "list").
But (back in the past), if the pilot hit the MPO switch while flying around "normally", the CLAW would not change, but it would do one of these partial re-initializations. Since the CLAW before and after the switch toggle are exactly the same, there would be no "transition" to smooth out. But, there is still stuff going on and the transition-smoothing filters are still triggered.

Which brings me to my theory. Assume the MPO switch was faulty and, while in the neutral position, was continuously bouncing between an indication of ON and OFF. In this case, the CLAW would be doing a "BI" with every "bounce". If the OFF-ON transition was once every 1 or 2 seconds, it would have no discernable effect on the CLAW's performance. If, however, it was more like 5 or 10 transitions every second, it really could have an impact. While I wouldn't even want to venture a guess as to exactly what that impact would look like, I can say it would not be good, that's for certain. At the very least, CLAW performance would be degraded. Depending on the flight condition, c.g, etc, the natural result from a degradation of the artificial stability system could very likely be a tendency to pitch nose-up.

So why did holding the MPO switch help? Well, I have to assume that as long as the switch was held, it sent out a steady "ON" indication, with no more bouncing back and forth between "OFF" and "ON". The CLAW would see one final transition to "ON" (when the pilot hit the switch), and it would do one more frame of re-initialization. But after that it wouldn't see any more changes, it would stop resetting itself, and would operate completely normally.

Obviously, this theory hinges on the faulty switch assumptions, which is why it would be great to know what inspections were done. It would also be a bit of a coincidence that he happened to press the one switch that was causing the problem (maybe he tried others first?).

Interesting thought study, nonetheless....

Digital FLCS didn't kick in until B40, right? No idea for sure but I suspect this was before that...I think the guy was ANG which means they would most likely not have the latest block in the early 90's.

Roscoe's suspicion is correct - it was a Block 10 A model.

Forum: F-16 Procedures

Notes, warnings, and cautions