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I have done numerous inflight engine restarts in the course of testing several different engines. However, each one of them was done with a full-up (that is, completely functional) engine and fuel system. The usual drill is to establish the conditions that you need for the test point. If it is going to be required, manually turn on the jet fuel starter and/or the emergency power unit. With that all set, shut the engine down by selecting cutoff with the throttle, wait for the engine to unwind to the desired rpm, and then position the throttle to idle or above for the start. As a result, the fact that I was going to do an airstart was never a surprise. I always knew ahead of time that the engine was about to quit.
Recently, I saw how the engine would quit in other than the normal airstart-test type of atmosphere. Although these tests were accomplished on the ground, I saw several things that I hadn't seen before, things I'd like to pass along. You can never know too much about the airplane.
I am sure that you are aware that there have been some as yet unexplained flameouts with the F110 engine. What we attempted to do was to try to induce as many potential malfunctions in the airplane as possible in order to shed some light on some of the past accidents/incidents. The airplane was tied down in the normal manner and then fully instrumented to record all the parameters of interest.
One large impression that I had was how tenaciously the engine will cling to life if it has fuel. On one test, we had artificially closed the master fuel shutoff valve to only five percent of capacity, that is, ninety-five percent closed. (There is no way that you can do this in your airplane without some really weird failure, or a plumbing change like we had for the purposes of this test.) The boost pumps were off and the refueling door was open so the system was depressurized. The engine was in idle and running just fine. The test point called for me to snap-accel the engine to ninety-five percent. The engine only briefly touched ninety-five percent, immediately rolled back to ninety-two percent, and hesitated there for a few seconds. It then rolled back to about eighty-seven percent for a few seconds. Subsequently it flamed out, but had an automatic restart accomplished in time to catch the rpm at eighty percent. It stayed there for another few seconds, and then flamed out again. The engine then auto-transferred to secondary engine control, or SEC, and got another auto restart at seventy-two to seventy-three percent. It maintained this condition for a little while and then flamed out again, with an auto restart in SEC at about sixty-five percent where it stayed for a while and then slowly continued to decay toward zero rpm. From this, and other similar runs, I feel that if the engine is operating properly, you have little fear of its quitting as long as the aircraft is providing fuel.
I have taken the time to go through our metal mockup here at the factory in no little detail. The fuel system is simple and straightforward, designed to fail toward an operate condition, should a malfunction occur. Hey, I agree with you that the fuel system diagram in the Dash One is really not too clear as to what fuel gets directed to which tank. The one that I have included in this article is a little easier to interpret because it was drawn with no lines crossing another line. It is a little strange though, since it ends up that the left side is on the right and the right side ends up on the left. If you take the time to look at the fuel routing from the farthest point in the system all the way to the engine, you will see that it is a very simple route. There are actually two separate routes in parallel with one being the siphon route and the other the powered, boost pumped/scavanged route. The pickup in each tank is carefully thought through, and there is little if any fuel that is not usable in the system – if you have all the boost pumps on. I really don't think the aircraft fuel system is at fault.
How about the engine? Once again, I really don't think that the engine is at fault. I can't say unequivocally that there is not some gremlin lurking somewhere in the engine just waiting to catch you unaware, but I don't think so.
What does that leave? Your fuel management – or the lack thereof. As I just said, the system is simple and nearly foolproof and is intended to be essentially automatic. However, if you leave the air refueling, or AR, door open, the external tanks will not transfer and you have in essence trapped all the fuel in the external tanks. You also partially degrade the siphon action from one internal tank to the next internal tank. It should be obvious: Don’t leave the door open! Flight Leads, what are you doing? Tell Blue Four to close the door. Wingmen, you don't do the Force any good by being bashful in pointing out to Lead that his refueling door is still open.
But there is no trapped fuel light in the aircraft to remind you; you must see this by monitoring the fuel indications (All the fuel indications! Not just the totalizer). Although the Dash One can be read to mean that you should leave the fuel quantity select knob in the normal position in order to be afforded the fuel trimmer and bingo fuel warnings, I suggest you reread the book and leave the knob on the external tank group that should be feeding at the time. And you might want to take another look at Maj. Steve Kniffen's excellent "Trapped External Fuel" article in the 4 July 1988 Code One.
Until the external tanks empty and you start burning internal fuel, there is no place for the system to pump fuel to keep things trimmed anyway. Plus, I hope you aren't flying a profile that has your bingo fuel occurring before the external tanks go empty.
OK, I'll get off my boxe du detergente. Pay attention to the fuel in the airplane and where it is all located and you should not be faced with any sudden silences as the engine quits with fuel still left on the totalizer.
If you do screw up, there are a couple clues that might save you (at the last possible moment). As the fuel gets so low in the reservoir that you are in danger of flaming out, the first indication is a wildly fluctuating fuel-flow meter. For example, if you are cruising along with a fuel flow in the vicinity of 4,000 pounds per hour, it will suddenly jump up to 8,000 to 10,000 pounds per hour, drop back to 4,000, jump to 7,500, and so on. This will continue for a short time, depending on the rate that you are using fuel, and then you will start to hear strange noises as the combustion becomes unstable in the engine. You will hear a low rumble as parts of the ejector nozzles become too lean to burn. As this continues for a while, the engine will start to roll back, sub-idle, and then inexorably fade toward zero, regardless of the throttle position you might have at the time. You should have long since noticed that the tanks were not feeding and the needles did not match what the totalizer was saying. You should have also noticed the forward and aft low-level lights and the master caution light.
The Dash One is fairly clear as to what to do once the fact that you have trapped fuel finally sinks into your consciousness. But a couple things are worth pointing out. Get the AR door closed! The system will not pressurize until you do. Make sure that you have the air source knob in norm or dump for the same reason. I would probably then accomplish steps five and six together. Step five gets around one of the possible failures in the fuel system where the airplane thinks that the centerline tank (phantom or otherwise) is still feeding and had not allowed the external wing tanks to start. Step six changes the pressurization schedule in the internal system and allows the external wings to fill the internal wings perceptibly faster. You are still faced with what now appears a maddeningly slow transfer rate from the internal wing to the fuselage and thence to the reservoirs. The turbine pumps provide about 6,000 pounds per hour, plus some help from the siphoning system, counting both sides (3,000 per side.) So, if at all possible, try to ensure that you are burning something less than 6,000 pounds per hour. If you catch it in time, you will probably survive to tell the story. And tell it. Maybe your candor will save an F-16.
But if you don't catch all the clues in time (over ten of them as I look around the cockpit from left to right) and get the fuel low enough that one or both of the reservoirs are empty, you are in real trouble. The engine has flamed out. Without the engine providing bleed air, you cannot get the fuel that remains out of the external tanks. If enough air has been going through the system, it might have been picked up in the separate line to the JFS and it might not stay running. You are in a world of hurt.
So. Am I making sense? If I am proven wrong, you will see it here in print. But I don't think the engine is at fault. Time will tell. There are only three failures within the fuel system that will preclude fuel from transferring from the external tanks and one of them is failed to the operate position. You must have power on the airplane in order for the solenoid valves on the open AR door to depressurize the system. Even with the door open, if you lose power, the system is fail-safe. The solenoid valve will close, the door remains open, but the system will repressurize. So unless something does turn up in the engine, I encourage you in the meantime to pay attention to where the fuel is in the airplane and make sure it is getting to the reservoirs. The engine will keep running.
Check Six. And the fuel gage . . . and the fuel sel knob . . . and the needles . . . and the AR switch . . . and the AR light . . . and the ECS knob . . . and the forward res light . . . and the aft res light . . . and the master caution light . . . and bitching Betty. Isn't there some clue in there somewhere? Unless something breaks in the engine, a flameout should never be a surprise.
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