correct as is Cold start, JFS switch

[skywalker22]

There are two accumulators inside the F-16 to start Jet Fuel Starter. If putting Jet fuel switch to START 1, it empties both - which is incorrect. Only when switching to START 2, should empty both at the same rate/time.

 

Edited July 6, 2024 by skywalker22

[Raptor9]

@skywalker22, the reason your engine would not start is because there is no fuel in the aircraft.

I added fuel, used JFS 1 position to deplete one accumulator, then manually shut off the JFS without moving the throttle to idle. Waited a moment for the engine to spool down, then tried JFS 1 a second time and was able to start successfully.

[skywalker22]

Strange, sometimes the accumulator just doesn't want to deplete at all. Seems like it has infinit pressure, at least trying offline (from mission editor). What do you say to that?

 

Edited July 6, 2024 by skywalker22

[Raptor9]

3 hours ago, skywalker22 said:

Strange, sometimes the accumulator just doesn't want to depete at all. Seems like it has infinit pressure, at least trying offline (from mission editor). What do you say to that?

I would say you have a misunderstanding of how the accumulators and JFS work. When you engage the JFS switch to START 1 or START 2, the hydraulic accumulators are used to the start the JFS, which is a small gas turbine like an APU. START 1/START 2 are essentially start switches for the JFS, and once the JFS gets up to speed the JFS Run light turns on.

However, once the JFS is spooled up and is self-sustaining, it no longer needs pressure from the accumulator to maintain speed because it is burning jet fuel. The difference being that unlike a separate APU, operation of the JFS is mechanically linked to the engine and starts spooling the main engine as well. This is what drives the RPM gauge on the right side of the instrument panel.

Running the JFS with the engine turning without moving the throttle to idle does not mean the accumulators have infinite pressure. In fact they don't. You can actually deplete the accumulators to the point that the JFS cannot even start. Try moving the JFS START switch to START 1 but moving it back to OFF a few moments later, prior to the RUN light coming on. Eventually the accumulators will deplete and starting the JFS will be impossible. But running the JFS alone does not mean the accumulators are constantly dumping pressure to spool the engine.

If any SME's want to jump in here to correct me, I'm all for it. But the tracks supplied in this thread show no behavior that is not intended.

[skywalker22]

The thing is, those two accumulators should in ideal conditions be only good enough for two engine starts. Now there, as I said in my previous post, are countless, realistically for no less then 10 starts. IRL those 2 accumulators can be used for max 2 attempts to spool up the JFS. I also tried what you said, but it's just not working as expected. Still engine can spool up each time I want. I also tried the same on some online server, and it was the same. 

So to sum up: to spool the JFS from zero rpm to 20-22% of the main turbo fan engine, it takes at least one accumulator, which gets discharged doing so. There will not be possible to spool up the JFS ones again with this accumulator. The 2nd one can be used as well (or in some special conditions, like hot and very humid, both accumulators are required, which DCS is not simulating). So there are max two attempts to spool the JFS. What are we also missing is option to refill these accumulators by the ground chief. So there will not be any issues with starting the JFS and and consequently the main turbo fan engine.

 

Edited July 6, 2024 by skywalker22

[skywalker22]

The JFS accumulators use a nitrogen precharge to store hydraulic pressure at 3000 psi that is used to spool up the JFS using a hydraulic start motor. When start 2 is selected it uses the stored hydraulic pressure out of both accumulators at the same time to start the JFS and a second start is not available without help from the crew chief on the ground. When the JFS fails to start the crew chief must pump up the bottles back up to the 3000 psi hydraulic charge using a pump built into the plane.

https://theaviationgeekclub.com/heres-why-starting-the-engine-on-an-f-16-viper-is-different-from-most-other-jet-aircraft/

Edited January 28, 2024 by skywalker22

[skywalker22]

Most important, which is not how it is with F-16 currently in DCS: If the F-16 doesn't start on the first two tries (JFS1 then JFS2) or on the first try with JFS2 then the crewchief has to pump the accumulators back up with a stainless steel tool called a T-handle.

https://www.f-16.net/forum/viewtopic.php?f=21&t=200&sid=4f14bdb55b6468f6371168210357ae2e&start=15

[Deano87]

Skywalker, the JFS always successfully starts within DCS, and as soon as the main engine gets above 12% RPM, which always happens when you run the JFS (regardless of if you actually start the engine or not) the accumulators start getting recharged.

What people are referring to I think is when the JFS fails to start, if that happens then you need the ground crew to recharge the accumulators.

 

[skywalker22]

On 2/2/2024 at 12:44 AM, Deano87 said:

Skywalker, the JFS always successfully starts within DCS, and as soon as the main engine gets above 12% RPM, which always happens when you run the JFS (regardless of if you actually start the engine or not) the accumulators start getting recharged.

What people are referring to I think is when the JFS fails to start, if that happens then you need the ground crew to recharge the accumulators.

 

Hmm, this makes sence, in DCS there are no engine failures simulated, at least not while starting it up, so JFS always start successfully. So to prevent accumulators getting recharged, JFS has to be switched off before engine reaches 12% RPM. Let me do a test...

... after the test, I successfully depleted both accumulators by just switching off the JFS just before engine's RPM reached 12%. It only took 1 cycle to deplete one and the other accumulator respectively (1st Start1 & then Start 2). It took two cylces to deplete both accumulators (Start 2). So this seems to work fine. We would only need a crew cheaf option to refill the accumulators (one day maybe).

 

Edited July 6, 2024 by skywalker22

[Spartan111sqn]

On 2/2/2024 at 12:44 AM, Deano87 said:

Skywalker, the JFS always successfully starts within DCS, and as soon as the main engine gets above 12% RPM, which always happens when you run the JFS (regardless of if you actually start the engine or not) the accumulators start getting recharged.

What people are referring to I think is when the JFS fails to start, if that happens then you need the ground crew to recharge the accumulators.

 

But then in flight if there is a flame out, and also JFS is discharged ( I understand from you that rpm has to ve below 12%rpm, and also on off before light come on), then you can recharge the JFS with windmill engine after a while, am I right?, or it is instantaneous when above 12%rpm?

On 2/2/2024 at 12:44 AM, Deano87 said:

Skywalker, the JFS always successfully starts within DCS, and as soon as the main engine gets above 12% RPM, which always happens when you run the JFS (regardless of if you actually start the engine or not) the accumulators start getting recharged.

What people are referring to I think is when the JFS fails to start, if that happens then you need the ground crew to recharge the accumulators.

 

But it shouldn't have sense, because that rpm is provoked by the JFS engine, then it means that it is generating same energy or more that the one that is using, cannot be.

[Raptor9]

@Spartan111sqn, there seems to be some confusion as to how the JFS works. The Jet Fuel Starter is not discharged and recharged, the JFS/brake accumulators are discharged to start the JFS itself. And the JFS, being mechanically linked to the engine, subsequently spools the engine so it can be started. Please review my second comment in the thread.

If you have a flameout in flight, that means that the F-16's engine was successfully started to get the aircraft airborne in the first place; and if the engine was successfully started, then the JFS/brake accumulators will have been recharged. If the JFS/brake accumulators have been recharged, the JFS can be used for an in-flight restart.

The premise of your questions is not logical. If the JFS/brake accumulators are discharged during the engine start sequence and never recharged, then the F-16 would never have any wheel brakes upon landing. Further, if the JFS/brake accumulators were, for whatever reason, not correctly recharged during the start-up sequence, the pilot would discover this when initiating taxi from parking when testing the nosewheel steering and brakes to ensure proper functionality, and abort taxi and the flight altogether.

[Spartan111sqn]

2 hours ago, Raptor9 said:

@Spartan111sqn, there seems to be some confusion as to how the JFS works. The Jet Fuel Starter is not discharged and recharged, the JFS/brake accumulators are discharged to start the JFS itself. And the JFS, being mechanically linked to the engine, subsequently spools the engine so it can be started. Please review my second comment in the thread.

If you have a flameout in flight, that means that the F-16's engine was successfully started to get the aircraft airborne in the first place; and if the engine was successfully started, then the JFS/brake accumulators will have been recharged. If the JFS/brake accumulators have been recharged, the JFS can be used for an in-flight restart.

The premise of your questions is not logical. If the JFS/brake accumulators are discharged during the engine start sequence and never recharged, then the F-16 would never have any wheel brakes upon landing. Further, if the JFS/brake accumulators were, for whatever reason, not correctly recharged during the start-up sequence, the pilot would discover this when initiating taxi from parking when testing the nosewheel steering and brakes to ensure proper functionality, and abort taxi and the flight altogether.

First of all thanks for your answer!, I ment that if in flight JFS is discharged intentionally, the way to charge it is with the windmill certain time, I guess 1 minute around 400kts. I tried intentionally to discharge it but without success. I am trying to reproduce an emergency procedure, also it seems that to re-start the engine with windmill is necessary to be below 20.000ft and below 400kts, but in DCS you can reignite at above altitude than that and any speed.

 

Just to know if such things are implemented with fidelity.

 

Thanks in advance for your feedback!