Question on afterburner operation.

[Retracting Head Ter Ter]

I never flew a bird with afterburners outside a PC sim so....

Can afterburners be engaged before the engines have reached full static/dry thrust? If an engine is 20000 dry and 30000 afterburning, can the pilot hit the burner button when the engine is still spooling up at 5000 dry thrust and get 15000 ASAP?

How are these things rigged up?

[Nied]

Well technically speaking, the afterburner is just a second stage combustion chamber that opens out to the back of the plane. Essentially fuel is dumped into the exhaust where it ignites and gives you the extra thrust. Technicaly speaking you could start dumping fuel into the exhaust before the engine is fully spooled and still get the boost from the afterburner but you wouldn't get full power since your only boosting half thrust. Of course with most new FADEC engines spool time isn't as much of an issue anymore.

Edited June 9, 2004 by Nied

[ewilen]

Regardless of whether it's technically feasible, do any airplanes allow engaging afterburner when not already running at full thrust?

Might engaging afterburner in some configurations (airspeed/AoA/thrust) cause problems?

[Syngyne]

can the pilot hit the burner button when the engine is still spooling up at 5000 dry thrust and get 15000 ASAP?

Is there an actual button for afterburner? Granted, I've only played PC flight sims, but I thought the afterburners kicked in after a certain position on the throttle.

[David Hingtgen]

Generally you must be at full throttle to engage afterburner. Like Nied said--partial throttle would only give partial thrust. Why do 50% throttle with afterburner and waste fuel, when you could just go up 80% throttle and get the same effect? I believe I read that either the Thunderbirds or Blue Angels had a mod done to allow afterburner to be selected at only 90% thrust. They're the exception though.

Afterburners are independent, you select them on when you want them.

Most modern planes do not simply have "on/off" but multiple stages. If you spend time looking up engine exhausts (like myself) you can usually see the individual burner rings. A high-end fighter like an F-14 will have 5 stages. Probably eaisest to see on a B-1B, having the physically largest afterburners---about the only plane you can visibly see the individual burner rings light up. (I think they go from inner to outer). 3 is the most common after that, then simply "on/off". And that is why there's not really a need for say 80% throttle and burner. Max thrust rating is based on max burner. If you've got 20,000 dry and 30,000 wet (which'd be 100% and zone 5 burner), then if you want 25,000 you just select like 100% throttle and zone 3 burner. Much more efficient than 80% throttle and zone 5 burner would be.

"Zone 5" is fighter-pilot speak for absolute maximum thrust. "Minimum afterburner" (zone 1) is very often used, especially the SR-71. The SR-71's engines are SO optimized for Mach 3, it often has thrust problems trying to refuel, and it'll turn on min burner on one engine, for a small boost in power. (They have such huge, slab, rudders that they can easily counter the thrust asymmetry)

Edited June 9, 2004 by David Hingtgen

[Retracting Head Ter Ter]

I know 60% dry and full afterburn would waste fuel.

But I was thinking of situations when the aircraft is flying on 30% dry thrust when an emergency requires power_right_here_right_now and the engine would take 5 seconds to spool from 30 to 100% dry thrust. Wouldn't hitting the afterburners in such a situation help? Even if its only 2000 or 4000 lbs out of the engine's potential 30000?

I think the throttles are designed such that the lever has to be pushed past the full military power setting into the afterburner zone right? Are they designed such that if the pilot slams it all the way forward (when the engine is at 20-30%), the burners stay unlit until the engine spools up to 100%? Or does the burners ignite once the lever is slammed past the full military power mark?

[David Hingtgen]

1. Jets have VERY high flight idles. 60% is not unheard of. Many a jet touches down with the engines run up to 70%. 40% is a typical "low" flight idle for a turbofan, with 50% being common for many military jets. If you're in an F-15 at 30%, you're either shutting down or starting up the engine, or are taxiing at ground idle.

2. Afterburners aren't "instant on". If you're in a plane with that slow of a spool-up, odds are the afterburner's pretty slow too. And once it's on, it'll take more time to get all the rings lit sequentially.

3. If you need power NOW, you slam the throttles full forward, and it'll do what it can as fast as it can. It'll engage the afterburner as soon as possible, but the power from spooling up will give more power, faster. Afterburning at idle won't give much more power

4. Throttles (like flaps and spoilers) have detents---it'll "stop" at various common points, and you generally either lift the throttle a bit to go over, or move it to the side to go around it, to get to the next point. Much like a car's gearshift, especially the "fancy" automatics. Or just push harder.

Afterburner is just another stop. Basically, a fighter jet's throttle will detent at start, ground idle, flight idle, mil, and max (afterburner). "Max" is zone 5 (assuming you have that many), so min afterburner is "move the throttle just a bit past mil". Fighters are not airliners, they are not too concerned with tweaking the throttle 0.1% at a time for max fuel economy. They generally fly/cruise at mil (full non-burner), and fight at max (afterburner). The F-16 was about the first jet with so much power it might actually want something less than full non-afterburning power...