Engines shut-off valve

[Pâte]

Hi guys.

 

Little question. How does the shut-off valve work exactly (aka FIRE light button) ? The way i understood is that it close the valve after the feed tank so engine is not provided with fuel anymore.

 

However even if the FF indicate 0, the RPM don't drop as much as when you switch the throttle to OFF. Where does this difference come from ? Some people came with an idea that could make sense: if the throttle is not on OFF position, the engine still use the fuel left in the lines, providing few RPM. But should be a quite short amount of time isnt ? I did some test and kept the 'high' RPM forever which doesnt make sense.

 

This question came because when practicing emergency engine lost in flight, i noticed that when the failure is simulated by pushing the FIRE light button the engine still at fairly high RPM (50). While if yu put the throttle then on OFF the RPM drop to the 12 for 350kt announced in the manual.

 

Thanks :)

Edited August 26, 2019 by Pâte

[Lex Talionis]

Weather starved from throttle cut off or from the emergency shut off, the eng will flame out the same and should windmill at the same RPM.

 

It is not as if more fuel in the lines extends the flame out process. The eng will simply run longer, then flame out the same as any other reason it is starved of fuel.

 

:)

[Flamin_Squirrel]

The NATOPS isn't explicit about the fire button doing this, but I wouldn't be surprised if pressing the button cuts off electrical and hydraulic connections. This would reduce load on the engine and might explain the difference.

[Pâte]

Weather starved from throttle cut off or from the emergency shut off, the eng will flame out the same and should windmill at the same RPM.

 

It is not as if more fuel in the lines extends the flame out process. The eng will simply run longer, then flame out the same as any other reason it is starved of fuel.

 

:)

 

This is what i thought initally but it doesnt happen like that in DCS, maybe a bug they need to correct then ?

 

 

The NATOPS isn't explicit about the fire button doing this, but I wouldn't be surprised if pressing the button cuts off electrical and hydraulic connections. This would reduce load on the engine and might explain the difference.

 

The NATO manual is quite clear :

 

" Actuation of the engine/AMAD fire extinguishing system can only be performed manually. The system is armed by lifting the guard and pressing the affected FIRE warning/extinguisher light. This also shuts off fuel to the engine at the engine feed shutoff valves and closes the crossfeed valve. "

 

This is why i don't understand that difference in the RPM.

Edited August 26, 2019 by Pâte

[Lex Talionis]

It only shuts off the fuel.

 

You can still rotate the eng via the x-bleed and thus rotate the AMAD with all of its services for EP redundancy considerations. Hyd pump is mechanical and can only be bypassed or "shear pinned ", it will only free up the AMAD if it shears however. Gen also has a sheer pin but unloads when the field is not excited. Regardless, the AMAD as a unit is what contributes to the majority of the drag on the eng and you probably won't see an appreciable increase with a wind milling N2 unless the AMAD drive shaft shears removing the entire AMAD.

 

NATOPS Nerd lvl 11 achieved :)

Edited August 26, 2019 by Lex Talionis

[bbrz]

This question came because when practicing emergency engine lost in flight, i noticed that when the failure is simulated by pushing the FIRE light button the engine still at fairly high RPM (50). While if yu put the throttle then on OFF the RPM drop to the 12 for 350kt announced in the manual.

Just tested and shutting down the engine with the eng fire switch or the throttle doesn't make any noticeable difference.

 

Tried in level flight at around 250kts and when using the fire switch there might be a very small delay until the engine starts to spool down, but that's the only difference. Windmilling RPM was around 15% in both cases.

[Pâte]

It only shuts off the fuel.

 

You can still rotate the eng via the x-bleed and thus rotate the AMAD with all of its services for EP redundancy considerations. Hyd pump is mechanical and can only be bypassed or "shear pinned ", it will only free up the AMAD if it shears however. Gen also has a sheer pin but unloads when the field is not excited. Regardless, the AMAD as a unit is what contributes to the majority of the drag on the eng and you probably won't see an appreciable increase with a wind milling N2 unless the AMAD drive shaft shears removing the entire AMAD.

 

NATOPS Nerd lvl 11 achieved :)

 

Not sure i understood sorry ^^ . How is this supposed to affect the difference of RPM ?

[Lex Talionis]

Million dollar question, does the eng seize (0 rpm) with loss of oil? .... or is the damage model such that it just flames out and windmills as well.

[Pâte]

Just tested and shutting down the engine with the eng fire switch or the throttle doesn't make any noticeable difference.

 

Tried in level flight at around 250kts and when using the fire switch there might be a very small delay until the engine starts to spool down, but that's the only difference. Windmilling RPM was around 15% in both cases.

 

Here a track.

 

- Both FIRE light pushed, RPM drop

- L throttle OFF, R throttle IDLE

- nose down to reach/keep 350kt

- L RPM went around 12 as supposed while the R was much higher.

IDLE_vs_OFF_TEST.trk

[Lex Talionis]

Pate

You probably won't see a differences in a wind milling eng unless the entire AMAD was removed from the rotating mass vie the shearing of the AMAD drive rod. Otherwise it doesn't really matter if various services on the AMAD are "turned off" or not. If the AMAD as a unit is still spinning with the eng, it is still dragging the eng in an appreciable way.

 

The AMAD is a relitively large, heavy, chuck of rotating mass.

 

Windmiling RPM will change with AS also, but not by much.

Edited August 26, 2019 by Lex Talionis

[bbrz]

Ok, there seems to be a bug.

 

Shutting down the engine at 450kts with the throttle results in a windmilling RPM of approx 12%

 

Shutting down the engine with the fire switch results in a windmilling RPM of 50% (same IAS)

 

At 250kts and below I didn't see any difference.

[Pâte]

Pate

You probably won't see a differences in a wind milling eng unless the entire AMAD was removed from the rotating mass vie the shearing of the AMAD drive rod. Otherwise it doesn't really matter if various services on the AMAD are "turned off" or not. If the AMAD as a unit is still spinning with the eng, it is still dragging the eng in an appreciable way.

 

The AMAD is a relitively large, heavy, chuck of rotating mass.

 

Windmiling RPM will change with AS also, but not by much.

 

Sorry for the stupid question but traducing give a result that may not be fully correct so to double check i didn't missunderstood. When you say that it shear, do yu mean that the unit is disconnecting from the AMAD rod letting it rotate freely ?

 

So that what could affect the RPM is if the AMAD still linked to some systems such as the hydraulic etc ?

[Lex Talionis]

Sorry for the stupid question but traducing give a result that may not be fully correct so to double check i didn't missunderstood. When you say that it shear, do yu mean that the unit is disconnecting from the AMAD rod letting it rotate freely ?

 

So that what could affect the RPM is if the AMAD still linked to some systems such as the hydraulic etc ?

 

The questions are all good. :)

 

Most rotating "things" in aviation that rely on engines to be driven have drives that are designed to shear off as to not hinder the engine if those "things" were to seize.

The AMAD is no different other than its relative larger size. It has a relitively large drive shaft, like those found in a rear wheel drive truck, that runs from the AMAD to the front of the eng, and is also designed to shear in half if the AMAD were to seize for any reason. This shearing of the drive shaft is the only way for the AMAD to be isolated from the eng.

 

Each of the individual accessories that are bolted to the AMAD, hyd pump, gen, etc, have shear shafts between themselves and the AMAD for the same reason.

[Pâte]

Copy thx for the clarification, wasn't sure about it :)

 

 

So that would mean that, regarding the result of the test maybe

 

The throttle on IDLE : the AMAD will shear off from the engine, maybe the fuel pump too.

While the throttle on OFF : would not ?

 

:huh:

 

Is the amad able to rotate on his own with windmill, or does it need the engine.

Edited August 26, 2019 by Pâte

[bbrz]

At which indicated speed did you made the tests?

 

Apparently there is a bug at higher speeds, e.g. above 400kias.

[Lex Talionis]

Copy thx for the clarification, wasn't sure about it :)

 

 

So that would mean that, regarding the result of the test maybe

 

The throttle on IDLE : the AMAD will shear off from the engine, maybe the fuel pump too.

While the throttle on OFF : would not ?

 

:huh:

 

Is the amad able to rotate on his own with windmill, or does it need the engine.

 

All this shearing is typically a catastrophic mechanical failure of the part. Under normal operation everything stays together and is shutoff through deliberate means. I only brought up the concept of shear shafts to give context for scenarios where windmiling rpm may be different.

 

In short: shutting down the engine either through the throttle cut off , or any other deliberate means (pilot induced) will result in the same appreciable total rotating mass dragging on the eng thus should be the same windmiling RPM. Only through some catastrophic failure of a part (in this case the AMAD) will enough parasitic rotating mass be cut from the eng allowing it to spin faster during windmiling.

 

The AMAD for all intent and purposes is permanently bolted to, and rotates with, the rotating assembly of the engine and is unalterable by the pilot.

Edited August 26, 2019 by Lex Talionis

[Rainmaker]

Copy thx for the clarification, wasn't sure about it :)

 

 

So that would mean that, regarding the result of the test maybe

 

The throttle on IDLE : the AMAD will shear off from the engine, maybe the fuel pump too.

While the throttle on OFF : would not ?

 

:huh:

 

Is the amad able to rotate on his own with windmill, or does it need the engine.

 

Some aircraft have the ability to electrically disconnect the some of the equipment from the engines the case of failure, typically, fighter aircraft (at least US made ones) cannot.

 

Anything like your generators, AMADS, hydraulic pumps, etc are considered 'accessories', they require the motor to turn them. Just like the wheels on your car, they need an engine and a drive shaft to make them move.

 

Below is an F-15, but it’s the same concept.

 

 

The silver piece he has his hands on is the shaft that connects the motor to the AMAD. They are bolted on as Lex stated above.

 

All that stuff they are looking at is the AMAD, your generator, and hydraulic pumps. That shaft in the picture above goes into the back of that and spins the AMAD which turns the rest.

Edited August 26, 2019 by Rainmaker

[Rainmaker]

The eng will simply run longer, then flame out the same as any other reason it is starved of fuel.

 

:)

 

 

Typically, and probably by design, they do. When they don't, it's a bit of a head turner and a 'crowd pleaser' :D

[Pâte]

So we can consider that this RPM difference is a bug then i guess. Thanks for the clarifications guys :) .

*

I'll report it