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The JFS (Jet Fuel Starter) on the F-16 is basically an APU that runs on jet fuel, but started by compressed air, from what I understand.

As an aside: Please correct this too, preferably with some reference. Is it correct that it is started on compressed air? And that it gets refilled, compressed again, when the engine runs above a certain RPM?

Main question:

Why doesn't the F-16 JFS keep running? If it runs on jet fuel, couldn't it keep running as long as the F-16 has fuel? I believe the JFS is shut down automatically once the engine is running, but if it weren't for this feature, it would just keep running. Is that correct?

Edit addressing some comments:

Why am I asking this? Why would I want to have the JFS running?

It is simply a question to help me understand how the JFS works. It is unclear to me why it has an inlet and exhaust port. This indicates to me it runs like a normal APU. Yet, I find various information indicating it runs on compressed air, some sources say it runs on hydraulic fluid. If it runs on compressed air or hydraulic fluid, why would it need both an inlet and an exhaust port?

It isn't that I would like to keep the JFS running on the low-mileage F-16 I have in my garage, rather that I don't understand the mechanics of the JFS and how it differs from a regular APU.

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    $\begingroup$ Why would one want the JFS to keep running? Like the starter in your car...it disengages and shuts down once the engine is spinning fast enough to maintain itself. $\endgroup$
    – WPNSGuy
    Sep 27 at 19:55
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    $\begingroup$ Since now-deleted answer aviation.stackexchange.com/a/101089/34686 won't be visible all, I'll repost the link included in it: theaviationgeekclub.com/… . Worth reading in full. Says JFS is shut down when main turbine reaches 40% but doesn't say whether shut-down is automatic or manual. Does indeed say JFS is started by compressed air (but runs on jet fuel.) Maybe poster will resurrect that answer after some editing? $\endgroup$
    – quiet flyer
    Oct 1 at 13:46
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    $\begingroup$ But it's not exactly clear what the real core of your question is. Are you asking the exact shut-down mechanism for the JFS? Or are you asking the purpose of shutting it down? (Or both?) An answer addressing the latter has been dv'd but as far as I can see it was a valid answer to the question. $\endgroup$
    – quiet flyer
    Oct 1 at 13:51
  • $\begingroup$ Continuing to run the JFS after main engine start would consume fuel, unnecessarily reducing the plane's range for no appreciable benefit. $\endgroup$
    – FreeMan
    Oct 4 at 18:52
  • $\begingroup$ @WPNSGuy Thank you. I've addressed your comment in the question itself. I should have made it more clear why I'm asking this. As I've explained in the question now, I simply don't understand how the JFS works, and this question might help me understand. $\endgroup$
    – AlphaCentauri
    Oct 7 at 9:32

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The flight manual is available online currently at, https://info.publicintelligence.net/HAF-F16.pdf

It states in Section 1-36 (pg 65 in the linked PDF),

The JFS is a gas turbine which operates on aircraft fuel and drives the engine through the ADG. The JFS is connected by a clutch to the ADG and only provides torque when required to maintain engine rpm. If the ADG is not able to rotate (i.e., seized engine), the JFS runs, but the clutch prevents it from rotating the ADG. The JFS receives fuel at all times regardless of the FUEL MASTER switch position. The JFS is started by power from two brake/ JFS accumulators used either singly or together. The brake/JFS accumulators are charged automatically by hydraulic system B or manually by a hydraulic hand pump located in the left wheel well.

Also in the following paragraph it is stated,

The switch returns to OFF automatically during a normal ground start at 50 percent rpm.

So it would appear the JFS is ground started by pneumatic pressure which drives hydraulic fluid through a hydraulic motor. That during a ground start, the JFS is automatically turned off.

For in-flight operations the manual states,

Once running, the JFS does not shut down until the JFS switch is manually positioned to OFF.

This is most likely because for an in-flight restart the JFS does not develop enough torque until below FL200, and it simplifies the instrumentation and procedures required.

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  • $\begingroup$ Welcome to SE.Aviation, great first post! $\endgroup$
    – Kenn Sebesta
    Nov 7 at 21:12
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Despite its name, the F-16 JFS doesn't consume jet fuel. It spins due to compressed air. There are two bottles of air at 3,000 PSI that are used to spin it, and it in turns spins the main engine until it gets to 20% RPM, at which point the throttle is set to idle and the engine starts. Normally you use one bottle, but if you select bottle 2 then both bottles are used. If the start fails then the ground crew has to pump it back up with a breaker bar, which I hear is not a fun job. It's only job is to spin the engine until it can start on its own. It does not have an electric generator like the vast majority of APU's, with the exception of those identified as "huffers" because all they do is blow air into the engine or air starter to get it going.

Think of it as the starter on your car, except it does not disengage when the engine starts.

A true APU can provide electrical power, as well as whatever starting means the engine needs -- air, hydraulic or electrical. For example, the APU on a CH-46 medium transport helicopter (now retired from USMC service) starts with hydraulic pressure (as opposed to air pressure on the F-16) from a reservoir charged by a hand pump to 3,000 PSI, and after it starts it provides electrical and hydraulic power for the rest of the systems on the helo. It can run until the fuel tanks are empty. Once the main engines start the hydraulic reservoir is recharged back to 3,000 PSI for the next start.

So the answer to the question is that the JFS is always turning when the main engine is turning.

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  • $\begingroup$ Thanks for your answer. I've edited my question to reflect why I'm asking this now (short version: I don't understand how the JFS works). Some sources say it uses compressed air, others, that it uses hydraulic fluid, others, that it runs on jet fuel, in all cases, to spin a compressor of some kind. Why does it need an inlet and exhaust port if it uses compressed air? Isn't the JFS directly connected to the gearbox of the engine? In that case, the main engine, obviously has a main inlet and exhaust? I don't understand why the JFS needs its own inlet and exhaust if it runs on compressed air. $\endgroup$
    – AlphaCentauri
    Oct 7 at 9:35
  • $\begingroup$ Are you sure the JFS doesn't run on jet fuel? Are you sure it doesn't start using compressed air, and then, runs on jet fuel? theaviationgeekclub.com/… "‘The JFS is a small jet turbine started by an onboard compressed air bottle. Unlike an APU though, the JFS is directly linked to the main engine, so independent operation is not possible." $\endgroup$
    – AlphaCentauri
    Oct 7 at 9:41
  • $\begingroup$ @AlphaCentauri I agree, the JFS runs on jet fuel but is started by compressed air just as similar micro jet turbines employed for starting the jet engine $\endgroup$
    – U_flow
    Oct 8 at 8:54
  • $\begingroup$ Actually, you’re both wrong. The JFS is a small gas turbine which runs on the aircraft fuel supply once started. However, it is started up hydraulically with pressure provided from a accumulator charged with nitrogen gas. $\endgroup$
    – Romeo_4808N
    Oct 8 at 14:29
  • $\begingroup$ Incorrect. Look at the aircraft pilot's handbook and video. When the air bottles are discharged the indicators that are monitored are the MAIN engine RPM and the Fan Inlet Turbine Temperature. The JFS is NOT a traditional turbine. And nitrogen gas does NOT qualify as a hydraulic start medium. $\endgroup$
    – Juan Jimenez
    Oct 8 at 15:36
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The Jet Fuel Starter (JFS) is designed to automatically shut off once the main engine has reached 50% of maximum RPM. Theoretically, once started up, and the shut down sequence, not triggered, the JFS will run and run until the a/c runs out of fuel.

Just a note: for all practical purposes, the JFS functions like a APU; it’s a small gas turbine engine which provides power to start the main jet engine with.

The start up sequence for the JFS is a pair of hydraulic accumulators controlled by a cockpit switch (only one accumulator is typically required to start the JFS). These accumulators maintain pressure via a nitrogen gas charge in side them. Once discharged during engine startup, they are recharged by the aircraft hydraulic system when the engine accessory gearbox and hyd pumps are operating. They can also be charged by an external hand pump in the event there is no charge left in the accumulators and the engine is not running (Apparently this manual charging is very laborious and a great way to piss off your crew chief!).

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According to this source

Unlike an APU though, the JFS is directly linked to the main engine, so independent operation is not possible.

However, the JFS is a jet engine by itself, only this engine is pneumatically started. Therefore the chain is:

  1. The JFS is started by compressed air
  2. The main engine is started by the JFS
  3. Once the main engine is sucessfully started the JFS is powered down

This powering down means that the JFS is not supplied with jet fuel anymore, as it would offer no advantage to do so. The main engine is more efficent anyways, and the JFS is too small to add any additional power output. Additionally, the wear and tear of the JFS is reduced this way.

As a last remark: I could not find a source stating if the JFS also stops spinning. I can imagine two scenarios:

  1. Either the VFS is rigidly connected and simply spins with the main engine
  2. Or the VFS is coupled for example by some sort of free wheel, meaning it would stop to spin when powered off

Unfortunately, I cannot provide an answer which of the two is true, however I think option 2 is much more likely, as it would reduce wear and tear of the JFS and also reduced friction. Also, the JFS starts the main engine to about 40%, therefore the JFS would spin pretty fast if it would not be decoupled, which would impact the max speed requirements of the JFS therefore making it more heavy/expensive.

Another interesting link is from a similar JFS of the Mirage fighter jet.

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  • $\begingroup$ Re "This would mean that the JFS is not shut down after the start of the main turbine, but rather continues running." -- no, actually from further down in the source you linked: "At around 40%, the JFS is shut down, as the engine can now spool up on its own." The source doesn't say whether the shut-down is automatic or done manually. The source may be using "directly linked" in a somewhat ambiguous way-- think of the starter in your car. In a sense it's "directly linked" but in a sense it's not. Can't run starter w/o it engaging the engine, but starter doesn't run whole time engine runs. $\endgroup$
    – quiet flyer
    Sep 30 at 13:02
  • $\begingroup$ Also-- the link you quoted says the JFS is started by compressed air. Not that it is powered by compressed air. Two different things. $\endgroup$
    – quiet flyer
    Sep 30 at 13:05
  • $\begingroup$ @quietflyer you are right, I thought the jfs was merely a turbine as in, expanding compressed air, but it is indeed a micro jet turbine. It is therefore started but not sustained pneumatically. But the "continue running" was meant as "it moves with the turbine". I edited to clarify. $\endgroup$
    – U_flow
    Sep 30 at 16:27
  • $\begingroup$ @quietflyer Thanks for you comments, I reworked my answer quite a bit to include your statements and also to make it more "right" $\endgroup$
    – U_flow
    Oct 1 at 16:50
  • $\begingroup$ @quietflyer is incorrect. The giveaway that the JFS is not an independent traditional turbine engine is that there are no instruments in the F-16 to monitor anything about its operation. $\endgroup$
    – Juan Jimenez
    Oct 8 at 15:37

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