No backup hydraulics when engine goes out in flight?

Yeah, they do. There is a couple of those.

Yeah, they do.

Figures...

By "accumulators" are not meant electrical batteries, but hydraulic accumulators.

These are pressure bottles with a rubber balloon inside, filled with nitrogen.

These balloons are pre-charged with the nitrogen with pressure.

When the normal hydraulic pumps operates, hydraulic oil is pumped into these bottle and compresses the nitrogen filled balloon. This way hydralic pressure can be stored.

Not sure if the AV-8B has such accumulators though.

Well DCS M2000 has control after engine is out... (I don't remember though if I had 0% fuel left or if there is an APU on the M2000)

Yes so now the question is at which RPM will the engine deliver power to the hydraulic system. :)

I'd guess that windmill RPM should be above 20% for the hydraulic system, etc. to keep working

... attempts should be initiated while windmill rpm is above 20 percent

...

Attempts made at greater than 250 KCAS within 10 seconds of shutdown from high power setting will increase the chances of a satisfactory start.

...

So with enough airspeed, the Harrier engine (which is not running anymore) should turn fast enough to produce enough hydraulic pressure?

I'd guess that windmill RPM should be above 20% for the hydraulic system, etc. to keep working

Base on the behavior of other aircraft hydraulics pumps, I would guess It would not produce the gallons per minute needed. I have been instructed that hydraulic pump provide flow, pressure is created when resistance is encounter, not sure the validity of that.

By "accumulators" are not meant electrical batteries, but hydraulic accumulators.

 

 

These are pressure bottles with a rubber balloon inside, filled with nitrogen.

These balloons are pre-charged with the nitrogen with pressure.

 

 

When the normal hydraulic pumps operates, hydraulic oil is pumped into these bottle and compresses the nitrogen filled balloon. This way hydralic pressure can be stored.

 

Not sure if the AV-8B has such accumulators though.

This is all conjecture in my part, but flight control accumulators normally are there to help with surges or provide pressure for an instant wile hydraulic pump caches on during high use of the controls. Would not be enough to control the aircraft from more than a second.

Base on the behavior of other aircraft hydraulics pumps, I would guess It would not produce the gallons per minute needed. I have been instructed that hydraulic pump provide flow, pressure is created when resistance is encounter, not sure the validity of that.

The hyd pumps produce a volume of oil that is pressurised to 3000 psi, a reserve of which is stored in the accumulators under pressure.

Pressures of 3000 psi are normal throughout the engine rpm range with no hydraulic system demands, so should be expected at Idle RPM = 28.4% @ 0ft MSL

This is all conjecture in my part, but flight control accumulators normally are there to help with surges or provide pressure for an instant while hydraulic pump caches on during high use of the controls.

This is the primary reason for hyd accumulators, however they also provide an emergency reserve in the event of pump or engine failure.

Would not be enough to control the aircraft from more than a second.

IIRC accumulator bleed off in the Hawk is about 4-5 min (IIRC similar to the A-10C in DCS).

This is seen at airshows, etc. IRL as, after engine shut down (and loss of hyd pressure), the stored accumulator pressure deploys the Hawk's RAT, after 4-5 min the accumulator pressure 'leaks off' and springs retract the RAT.

In the case of the AV-8B the Hyd 2 accumulator will provide about 3 cycles (a cycle is from neutral to 3° L to 3° R and back to neutral) of nosewheel steering if both hydraulic pumps are lost.

Given a flame out decent from 30,000 ft might take ~7 min / ~40 nm, hydraulic pressure doesn't need to be maintain for long if the engine fails to relight.

With a flame out engine windmilling at 20% for an airstart, there's a good chance hyd pressure is maintained but it's pure speculation on my part, due to pilot needing to maintain control as he attempts the airstart.

Accumulators are both a buffer and a reserve. The AV-8B manual makes reference to the ability to turn the nose wheel back and forth about 3 times on its accumulator reserve. That's not a lot but it's not a tiny amount either. The fact the manual states that a dead stick landing is not permitted instead of impossible suggests there is some chance that control is still available that would tempt a pilot to try it. If engine failure resulted in loss of control within single-digit seconds the manual would point this out explicitly as this is critical information.

I don't know of any airplane that supplies significant hydraulic pressure under windmill. The pump could very well be quite non-linear with RPM or purposefully disengage itself when the engine is not running (to reduce resistance during engine start). It's been said that the A-10 doesn't produce any useful hydraulic power under windmill.

Accumulators are both a buffer and a reserve. The AV-8B manual makes reference to the ability to turn the nose wheel back and forth about 3 times on its accumulator reserve. That's not a lot but it's not a tiny amount either. The fact the manual states that a dead stick landing is not permitted instead of impossible suggests there is some chance that control is still available that would tempt a pilot to try it. If engine failure resulted in loss of control within single-digit seconds the manual would point this out explicitly as this is critical information.

 

I don't know of any airplane that supplies significant hydraulic pressure under windmill. The pump could very well be quite non-linear with RPM or purposefully disengage itself when the engine is not running (to reduce resistance during engine start). It's been said that the A-10 doesn't produce any useful hydraulic power under windmill.

15s emergency gen will get the cracking valves opened and the emer gen spinning somewhere between 13-19% on a single motor operating utility pressure. You can get the stabs moving a little bit on that amount, possibly manualy flying it would be a pretty big feat. Dry motoring with a JFS assist will get you in the low 20s RPM % wise.

So with enough airspeed, the Harrier engine (which is not running anymore) should turn fast enough to produce enough hydraulic pressure?

I think you under estimate the sheer force required to run these systems. In the Hawk for example even with the engine running it wont charge the hydro system at less than 60% of RPM. Anything below that and the system will discharge with use even with the engine running

Windmilling after an engine failure (assuming the engine could windmill after the failure)I strongly doubt wouldn't produce enough

Pman

On 7/20/2018 at 11:34 AM, Pman said:

I think you under estimate the sheer force required to run these systems. In the Hawk for example even with the engine running it wont charge the hydro system at less than 60% of RPM. Anything below that and the system will discharge with use even with the engine running

Exactly. It's not a large margin for functionality, so maintenance is the way to go for all systems.

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Alexandra from Somac Hydraulics