Loss of total Hydraulic Pressure with engines out/off*too fast/ no windmilling

[Badger1-1]

With no fuel, engine damgage etc. your Hydraulics will go to zero in an instant. 

Apparently the F4 we have really didnt have any emergency pumps to avoid that (manual states later models had it) but the bug for me is, that its instant.

With no loads on the system, standing on the ground you should not see an instant drop in pressure. Usually you can bleed it of by moving the stick a bit.

 

Manual states windmilling will provide Hyd power in flight, but again. Its not working, direct loss of control is the result.

Edited May 25, 2024 by Badger1-1

[Schwokol]

+1

I also noticed that

[Cobra847]

Thanks, this is odd as this system should be kicking in fully. We'll take a look!

[Diesel_Thunder]

16 hours ago, Badger1-1 said:

With no fuel, engine damgage etc. your Hydraulics will go to zero in an instant. 

Apparently the F4 we have really didnt have any emergency pumps to avoid that (manual states later models had it) but the bug for me is, that its instant.

With no loads on the system, standing on the ground you should not see an instant drop in pressure. Usually you can bleed it of by moving the stick a bit.

 

Manual states windmilling will provide Hyd power in flight, but again. Its not working, direct loss of control is the result.

 

That emergency pump you refer to is known as the "APU", and isn't an APU as we know it on newer aircraft. It was an electric powered hydraulic pump that only activated when the pressure in the PC1 (left engine) system fell below 1,000 PSI, and only provided 1,800 PSI to the PC1 stabilator controls. The catch here is it is powered by the right 115 VAC bus (right engine), and controlled by the main 28 Volt DC bus. So if you lose electrical power, that pump will not activate. It will also not activate in the event that the PC2 hydraulic system fails.

Very limited use with the "APU" and not many aircraft were fitted with it.

[=475FG= Dawger]

In brief testing in flight, flight controls are functional above 54% engine RPM. Below that they are off. 

[Badger1-1]

3 hours ago, Diesel_Thunder said:

That emergency pump you refer to is known as the "APU", and isn't an APU as we know it on newer aircraft. It was an electric powered hydraulic pump that only activated when the pressure in the PC1 (left engine) system fell below 1,000 PSI, and only provided 1,800 PSI to the PC1 stabilator controls. The catch here is it is powered by the right 115 VAC bus (right engine), and controlled by the main 28 Volt DC bus. So if you lose electrical power, that pump will not activate. It will also not activate in the event that the PC2 hydraulic system fails.

Very limited use with the "APU" and not many aircraft were fitted with it.

Good to know! but again, my issue is, that its an instant drop. I never saw a Hydraulic system in my life loose pressure from 3000psi to 0 in .1 seconds.

Even with Pumps of in a real AC, it remains at the Px a bit, so you could at least steer a bit before it goes out.

I flew some planes so far in my life, no F4 tho so maybe shes different

Edited May 26, 2024 by Badger1-1

[=475FG= Dawger]

1 hour ago, Badger1-1 said:

Good to know! but again, my issue is, that its an instant drop. I never saw a Hydraulic system in my life loose pressure from 3000psi to 0 in .1 seconds.

Even with Pumps of in a real AC, it remains at the Px a bit, so you could at least steer a bit before it goes out.

I flew some planes so far in my life, no F4 tho so maybe shes different

 

Hydraulic systems can only retain pressure with pumps off through the use of accumulators. Generally, accumulators are used for emergency braking purposes on aircraft. Its quite likely the Phantom doesn’t have an accumulator although I really don’t know. It would be unnecessary weight on an aircraft with a tailhook and if there isn’t another reason to install one ( shock absorbtion ) there won’t be one. 
 

EDIT: There is an emergency brake accumulator on the Phantom. I don’t know if it is connected in any way to the normal hydraulic systems. 

Edited May 26, 2024 by =475FG= Dawger

[Diesel_Thunder]

6 hours ago, =475FG= Dawger said:

Hydraulic systems can only retain pressure with pumps off through the use of accumulators. Generally, accumulators are used for emergency braking purposes on aircraft. Its quite likely the Phantom doesn’t have an accumulator although I really don’t know. It would be unnecessary weight on an aircraft with a tailhook and if there isn’t another reason to install one ( shock absorbtion ) there won’t be one. 
 

EDIT: There is an emergency brake accumulator on the Phantom. I don’t know if it is connected in any way to the normal hydraulic systems. 

 

There's a few accumulators around the aircraft. One for the wheel brakes, another to blow down the gear, another for the flaps, and one more for deploying the Ram Air Turbine (RAT) on the F-4's that were fitted with those. The accumulators get pressurized by the respective hydraulic systems, and also get a charge of compressed air from the pneumatic system, also 3,000 PSI.

[=475FG= Dawger]

2 minutes ago, Diesel_Thunder said:

There's a few accumulators around the aircraft. One for the wheel brakes, another to blow down the gear, another for the flaps, and one more for deploying the Ram Air Turbine (RAT) on the F-4's that were fitted with those. The accumulators get pressurized by the respective hydraulic systems, and also get a charge of compressed air from the pneumatic system, also 3,000 PSI.

The question is if any pressurize the associated system without manually opening an emergency valve. If they don’t, there won’t be any pressure on the system without a pump running. 
 

 

[Smyth]

The RL T.O. recommends a glide airspeed of 215 knots and suggests that will allow enough hydraulic pressure for control at any altitude under 50kft.

I just did a simple test shutting off the engines at low altitude around Mach 1. Power and controls die suddenly as soon as RPM goes below 55%, while the aircraft is still traveling over 400kts. RPM then stabilizes at 40% while maintaining 400kts on the way in to crash, so 400kts at sea level isn't nearly enough to maintain control. Surely this isn't behaving as intended?

Short .trk attached.

F4e_no_windmill.trk

[Zabuzard]

There was a bug that linked a few things relevant for the control surfaces to electric power. Has been fixed recently, so engine-off glide-behavior should be better

[gabuzomeu]

HI, there is an instant action (Persian Gulf, Divert) where engine loss leads to fast/immediate loss of controls. 

Zabuzard, is the fix you mentioned in game already?

[Zabuzard]

1 hour ago, gabuzomeu said:

Zabuzard, is the fix you mentioned in game already?

The fix was made 1st June. Remind me, when was the last update?

[=475FG= Dawger]

I tested it 2 days ago and the flight controls don' quit until somewhere less than 25% RPM's. I didn't bother with the exact number, just saw it was radically different and called it good.

[Dentedend10]

Yep, I also noticed the difference in that instant action in that I can actually fly now with both engines out. although I still haven’t made it to the divert field

[Flamin_Squirrel]

Apologies for a bit of a bump...

Single engine doesn't result in insta-death now. However, trying the emergency landing mission on the persian gulf map (in the dark) is made somewhat more challenging by the fact electrical power dies quite quickly. That doesn't seem right?!

[Zabuzard]

Apologies for a bit of a bump...
Single engine doesn't result in insta-death now. However, trying the emergency landing mission on the persian gulf map (in the dark) is made somewhat more challenging by the fact electrical power dies quite quickly. That doesn't seem right?!

Without engines you have no power, that is correct behavior. With a single engine that runs at full RPM, you should have plenty of power unless you got the BUS TIE warning.

Id suggest to use the white "thunderstorm" light (white floodlight). It is battery powered for exactly this reason :)

Otherwise a track or at least more details of what you are encounting would be useful

[Flamin_Squirrel]

Hmmm. Tried the mission again. The scenario is a left engine failure. The post mission failure lists a right engine oil leak too, but that remains running for long enough to land at least.

I don't get a BUS TIE warning, but power dies after a couple of mins. Operating engine at MIL power.

[Zabuzard]

Hmmm. Tried the mission again. The scenario is a left engine failure. The post mission failure lists a right engine oil leak too, but that remains running for long enough to land at least.
I don't get a BUS TIE warning, but power dies after a couple of mins. Operating engine at MIL power.

Can you send a track file? Then we can easily tell you what's going on :)

[Diesel_Thunder]

On 7/31/2024 at 2:49 PM, Flamin_Squirrel said:

Hmmm. Tried the mission again. The scenario is a left engine failure. The post mission failure lists a right engine oil leak too, but that remains running for long enough to land at least.

I don't get a BUS TIE warning, but power dies after a couple of mins. Operating engine at MIL power.

Doesn’t matter if the engine is at idle or full power, the gens are on constant speed drives (CSD) so the AC electrical power should be consistent. When an engine fails, the generator will drop offline around 53% RPM (it varies engine to engine) as the CSD no longer has enough RPM to work with.

I’ll have to look in the engine book again, but fairly sure the CSD’s need good oil pressure to function properly. Which could explain why the right engine gen dropped offline too.

[Flamin_Squirrel]

Ah interesting! Thanks. 

[=475FG= Dawger]

2 hours ago, Diesel_Thunder said:

Doesn’t matter if the engine is at idle or full power, the gens are on constant speed drives (CSD) so the AC electrical power should be consistent. When an engine fails, the generator will drop offline around 53% RPM (it varies engine to engine) as the CSD no longer has enough RPM to work with.

I’ll have to look in the engine book again, but fairly sure the CSD’s need good oil pressure to function properly. Which could explain why the right engine gen dropped offline too.

The HB manual addresses the CSD and oil leaks

Quote

The Engine Oil system is used for lubrication, variable nozzle positioning and constant speed drive unit operation. The standpipes which supply the three systems utilizing engine oil are in the reservoir such that the pipe for the constant speed drive unit is the highest, the one for the nozzle control is the next highest, and the lubricating system pipe is the lowest. This arrangement is to prevent a critical system failure if one of the circuits leak. If a leak in the constant speed drive unit would occur it will probably cause a failure of that system only, while a leak in the nozzle control system may cause failure of that system and the constant speed drive unit. 

 

[Stickler]

In 2.9.15.9509, shutting down both engines in flight will cause all hydraulic indications to drop to zero psi due to the generators dropping offline.

If able to maintain windmilling engine RPM above the generator cut-out speed (53-54%), as stated above, indications will remain available and show that hydraulic pressure is unaffected by the shutdown; PC-1/2 and Utility remain at about 3,000 psi.

In any case, the effectiveness of the primary control surfaces is not compromised by the shutdown. Even at very low speed with engine RPM near zero, the aircraft retains full maneuverability, only affected by the handling degradation due to high AoA.

This may be related to the issue that even the airflow from the starter generator is currently enough to maintain 3,000 psi without a demand on the flight controls. With full demand (constant stick wipeouts), the starter generator is sufficient to retain approximately 2,000 psi.

Bottom line: The current implementation of the hydraulics system during a double-engine failure may or may not be realistic depending on whether the real F-4E's system was able to retain flyable hydraulic pressures at very low engine RPM. Another matter is why the complete shutdown of only one engine (0%) does not cause the affected PC-1/2 system's pressure to drop to zero even with high demands on the control surfaces.

Edited April 23 by Stickler