Forum: F-16 Procedures

How long can an F-16 stay inverted? I would like to know "according to the book" and also, if you have much knowledge, how long until an actual problem occurs? There is only one boost pump mounted on the top of either the fwd or aft side, depending on model, so would it cause and imbalance as well, possible trapped fuel?

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Joined: Feb 01, 2007 - 02:38 AM Posts: 0 Status: Offline

No more than 30 seconds, at which time the OIL PRESS light will illuminate, and a fault is declared.

The viper can fly indefinitely while inverted. It can also extend and retract gear inverted, contrary to popular belief. It cannot however, do these things in zero or negative-G flight.

Inverted does not mean negative G, and vice versa.

STBYGAIN, I'm sorry, but you're gonna have to explain your answer. First of all, you will have a trapped fuel indication because the external tanks cannot tansfer while inverted, sure it can still fly, but fuel will be trapped. You can have a lot of trapped fuel if you were to stay inverted for too long, that can cause an imbalance. Again, it can fly, but it will have problems.

So my question is, how long with out problems? I would like to know where you recieved your information. Thank you for your response!

While STBYGAIN's answer seems to be theoretically correct, it is in practice not possible to fly inverted and pull 1 G for more than just a few seconds. Assuming that inverted is defined as wheels up relative to the earths mean surface angle, you need to be on a planet much smaller than the earth or the speed is high enough to enable you to pull 1G+ inverted without hitting the ground. Only the space shuttle goes that fast (in space orbit), but it still does not pull 1G, so they had to figure this out during the design stages.

To clarify STBY's response, inverted with positve G's applied would negate the no oil problem, as well as fuel being in the wrong place in the tanks. As for how long, fuel is now trasfered by pumps, so in later blocks the time it can be inverted is longer than earlier blocks as you stated in your post. However air pressure pushes the fuel from the external tanks into the airframe, there are no pumps in the tanks that I am aware of. Maybe one of our fuels guys can correct me on this one.

So clarify your question, do you want to know how long inverted with zero or negitive G load, or do you want to know how long with positive G load?

The fuel in the external tanks will stop transfering while inverted. Why? There is a float switch in the vent box, when the float is up, it closes the transfer valve in the externals, so, when the float is upside down, the float is "up", so the external tank transfer valves will close. The reason why? If the external tanks kept transfering, the jet would vent fuel because the float in the wing would be in the "up" position too, causing the externals to keep transfering, causing the wing to overfill, causing the jet to vent. As far as negative G's and such, I don't know too much about, but basically with normal conditons, with gravity affecting the system, that is what i'm concerned with. Even with the syphoning effect, there would still be trapped fuel in most of the fueselage as well as the externals. What do you think?

Joined: Feb 01, 2007 - 02:38 AM Posts: 0 Status: Offline

In a negative G situation I believe the engine oil system would the more limiting to inverted flight than the fuel system.

If you run out of fuel, you can always restart.
(Granted you had the altitude to recover from the inverted flight!?)

Run out of oil (for too long), the motor will seize, and you're through!

Negative G, the jet can still suck fuel and oil. At less than a G, the fuel and oil "pickups" float in "mid-air" with the fuel and oil and may not "suck" (sts) oil/fuel....

As I have no personal knowledge of the F-16's design and construction, I have a question based on my limited experience with aircraft. I've built and flown a couple of R/C planes, and I know that the fuel pickup in the tank is commonly on the end of a flexible fuel line with a weighted peice on the end of the line (or "clunk" as I've heard it called). This way, the "clunk" will follow the fuel if the plane is inverted for long periods of time so the engine doesn't quit due to fuel starvation. I would think the designers and engineers would want a fuel and oil system that would operate under virtually any aircraft attitude, considering this is a combat aircraft and not meant to merely fly straight and level. Can anyone tell me why this isn't the case with the F-16 or other fighters and attack jets? Confused

Joined: Feb 01, 2007 - 02:38 AM Posts: 0 Status: Offline

The oil tank has internal "Inverted" pickup and breather tubes. These tubes will permit the oil system some function during "negative G" flight. It DOES NOT use a [Link pending approval]

Even with this the engine is limited to 30 seconds of operation at MIL inverted, or 10 seconds of AB.

The reduction of time at AB would in part be due to the additional heat generated by the AB that would radiate to the turbine/turbine bearing, thus increasing it's operating temperature. This bearing would overheat due to the lack of "proper" oil flow at MIL/AB respectively. Don't forget the two primary functions of the oil system are lubrication and COOLING. Even if you get flow, if you don't have enough, or it doesn't flow PROPERLY it will not cool the bearings. Jet engines don't use a very high pressure oil system, but they are a high flow to maintain even bearing temperatures.

Yes there is oil flow to the bearings during Negative G flight, but the system is not designed to operate under those conditions for lengthy times. The tank has some provisions as I have stated, but the rest of the system does not. (Bearing Cavities, or the Gearbox)

As for the fuel system limitations you'd have to talk with a tank [Link pending approval] Smile

(I did say that with a [Link pending approval])

That makes sense. If there isn't ANY oil flow the bearings, you get a seized engine and an up close demonstration of the operation of the ACESII ejection system! Definitely not the desired way to end a mission!

I guess negative G's aren't usually endured for too long anyway. I remember oil being used for cooling from my private pilot groud school. Also, a fully inverted fuel and oil system like the ones used by airshow aerobatics planes like Sean D. Tucker's "Challenger" would add cost and weight which are both mortal enemies of fighter planes. More than a few people have told me that everything in aviation is a compromise, like more weapons means more weight and drag, which means less range.

Ya know, I never realized there was quite this much involved in operating these aircraft. Thank god we have pilots and maintainers as good as we do! Very Happy

Active Member Joined: Apr 08, 2004 - 10:40 PM Posts: 203

I have never heard of a fuel imbalance caused by inverted flight, even if the float in the vent box was raised, it would only shot off the external tanks, the wings would feed in fine, Being iverted really should not have any affect on your fuel system though, as the internal tanks are preseeurized to 4.6 to 6.4 psi and your externals are 19 to 24, so if you go inverted your air pressure will basically do one of its jobs and keep the fuel from slushing around and closing the "float" in the externals. BTW, the float in the vent box is not suppost to close by just turning it upside-down, it is suppost to be closed by rising fuel.

but, thats in a perfect world I guess. Smile

Tankrat

tankrat, i've never heard of an imbalance caused by being inverted either, but i'm sure thats because they do not stay inverted that long. I think there would be an imbalance if he did stay inverted long enough because all the pumps and manifolds that will "suck" fuel, are all mounted at the bottom of the tank, except for the pump mounted to the top of the foward res. , so, the fuel would be trapped in all the tanks except the foward res. so you would have a trapped fuel indication and imbalance. correct me if i'm wrong please. Also, the float in the vent box NEEDS to go upside down, if I didn't, the jet would vent fuel. I guess i'll have to ask a pilot to find out the book answer.

Active Member Joined: Apr 08, 2004 - 10:40 PM Posts: 203

The only time that float needs to raise is because of a problem in the refuel/transfer system, In the event of fuel venting the float will raise "ONLY" to shot off the externals. As for pumps not sucking fuel, please remember thats one reason your tanks are pressurized, your pumps are going to pull the same either way.

Forum: F-16 Procedures

How long inverted?