Forum: F-16 Procedures

Hi everybody,

Back with a question about the maximum altitude at which an engine restart can be made: should it be around 40.000 ft, like the old F-100 "Hun" ? Even though I know most reasons for such a flame out/compressor stall are pretty rare as far as the Viper is concerned.

I also wondered how many times the afterbuner could be engaged in a single mission, fuel consideration notwithstanding ?

Bye you all from Paris...

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The AB can be lit as many times as the pilot wants. In the old -100 engines, before there was SEC, they had a backup mode called BUC. Ask Gums about this. If you cancelled AB and then went for it again within a few seconds, the engine would compressor stall, causing a large bang that would knock your feet off the pedals and shoot flames forward from the intake. Wouldn't cause damage but still probably not the thing you would want to do.

The DEC engine logic will do everything it can to start the engine at any altitude, very little pilot action is required. All that's required for jet propulsion is compressed air, fuel, and spark. After the fire goes out, the DEC (or DEEC for PW) will do what's required for a relight. If my motor quits at 40,000 feet, I'm looking to drop my stores, get a good glide ratio, and find a long piece of concrete. If the motor relights itself during that time then great, otherwise I have bigger things to worry about.

STBYGAIN wrote:

In the old -100 engines, before there was SEC, they had a backup mode called BUC. Ask Gums about this.

Ugh!....BUC airstarts.......bringing back.......nasty nightmares...must......wake up! Laughing

Sir Obi-Gums could speak more about real-world BUC airstarts but in the sim they were always a wicked touchy procedure. I always believed the engine model had narrower tolerances than the real thing. They could be done, but you had to be smoooooooooth with the throttle movements. One little hiccup and the motor would hot start and ya had to start all over. Evil or Very Mad

STBYGAIN wrote:

The AB can be lit as many times as the pilot wants. In the old -100 engines, before there was SEC, they had a backup mode called BUC. If you cancelled AB and then went for it again within a few seconds, the engine would compressor stall.

Thanks STBYGAIN for your answer.
The few things I know about the backup controller come from J.B Dryden's famous articles, published in Code One Mag. almost two decades ago now : the BUC is a simple back-up system for the main fuel control, allowing easier air starts, kind of a "kitchen faucet".

Augmented turbofans not fitted with DEC systems (among which the
-100/200 series) appear to be more touchy than other engines (turbojets and modern turbofans alike) when hitting the AB switch on/off in quick succession, as there is an almost direct route for shockwaves between the afterburner back to the LP compressor and fan rotors.

You say a compressor stall could occur in such a "hard light" situation : but what power did pilots start from when AB was engaged anew ? Was that Mil or Idle ? Engine rpm and fan turbine inlet temperature (FTIT) were not the same in both cases. Could such stalls happen whatever current power, and even after say a 10-second lapse ?

I think going from a low rpm condition to AB was a more understandable risk to take in combat than becoming a sitting duck with a stalled engine (with possible stagnation to come) while engine rpm was still high. Otherwise I'd better understand what the term "teething troubles" could mean for Pratt & Whitney, especially since the F-15 A/C fighter shared the same powerplant until the -220/220E arrived on the scene. I get now all the importance for early Vipers pilots to be able to detect all possible cues of such events.

Waiting for any comment on that...

Bye. Very Happy

Ah, Joe Bob and Semper Viper... I still have those articles.

Please don't hold my post as religion, as I wasn't a suck and blow guy, er, an engine troop! Wink

But yeah snapshot, I do believe the P&W-100 hard burner lights came from quickly taking it from a lower throttle setting to burner. The fuel sprays into the exhaust as advertised and nothing happens...Finally, the torch ignitor pulls its head out of its @$$, and the fuel goes BOOM! The shockwaves pretty much instantly stall the compressor. In the F-15As and Bs, it had a tendency to occur at a high angle of attack. Then the DEEC came along, and there was also a quartz window added for the flame sensor, and the problem pretty much went away. I haven't heard of hard starts happening in a long time. It's cool to watch a hard start though. It'll rock the hush house, and rattle the fillings out of your teeth Shocked

Hard Burner Starts...aka, "How to make your F-15 sound like an F-106!" Mr. Green

Beers and MiGs were made to be pounded!

A bit more info for you too surrounding Block 50's.....

The F-16C/D Block 50 is powered by the F110-GE-129 low bypass turbofan engine. The engine is a twin-spool system that has a high overall compression ratio which provides a very high thrust to weight ratio, this design includes:

Variable geometry for the air path control
Annular combustor
Mixed flow augmentor
Variable area convergent/divergent exhaust nozzles
Engine systems that incorporate secondary (backup) functions for emergency us

The Digital Engine Control ....AKA... (DEC) is the critical component of the PRI engine control. The DEC is an engine mounted, fuel-cooled solid state digital computer that controls both the engine and the augmentor (AB).

Augmentor Fuel Control (AFC) is a fuel-operated electromechanical (that's a mouthful huh) control that regulates fuel flow to the AB working with the DEC.

Main Engine Control (MEC) is another electromechanically control that provides various control functions in both PRI and SEC (Primary and Secondary).

Primary Engine Operation (PRI) provides unrestricted engine operation throughout the entire flight envelope.

Some Engine Operation Characteristics.....

Since the DEC maintains constant idle thrust and minimum bleed air pressure, RPM's varies with temperature and pressure altitude (the higher temp or pressure altitude results in higher RPM's).

But I am getting a bit too technical for some, others know this as good or better then I do....

Forum: F-16 Procedures

In-flight engine restart