Unfeathering accumulator

[Timothy]

Is there any way to check the functioning of the unfeathering accumulator either before flight or as a part of the power checks?

W

[rustle]

I know one way to test it, but if it doesn't work you have a problem ;)

[Timothy]

I know one way to test it, but if it doesn't work you have a problem

Helpful as ever! :p

The reason I asked is that it failed on my CofA test flight, which didn't matter as a touch on the starter was enough to get it going, but I would like to know if it's going to work if I need it.

W

[I Fly]

I don't know what particular aircraft you fly. The purpose of the accumulator is to store oil pressure. If the engine is turning it is producing oil pressure and the accumulator can not do it's job. In flight when you use the accumulator to unfeather the prop, it only moves it a little bit so it will start windmilling. The turning of the engine produces the rest of the oil pressure to get it back into fine. On normal shutdown the centrifugal locks prevent it from feathering. The only way to test it, would be to start the engine, run it at the feathering test RPM, feather it, shut it down and then try to unfeather. The engine produces lots of vibration when you try and start it with the prop feathered or partially feathered. I suggest you would create a more dangerous situation. As you said "the starter got it going again". Once upon a time we did not have unfeathering accumulators and still survived.

[Timothy]

I

I don't know what particular aircraft you fly.

Aztec.

The rest of your explanation is what I had thought, which is why I was surprised that the engineer told me that the accumulator hadn't worked because the dome pressure of the Nitrogen had fallen.

What's that about?

Oh, and yes, I know that the accumulator is a "nice too", but I carry the weight and I pay for the repairs, so it would be nice to know it worked!

W

[Windy Militant]

W
An accumulator can take a number of forms but simply put its a can with a balloon in it. The idea is that you put a compressable medium either air or nitrogen into the balloon to a given pressure. When the engine runs, oil is pumped in and compresses the gas. If for any reason the pressure from the engine drops the gas expands pushing the oil back out into the system. This can then be used as an emergency supply or to prevent pressure drops when different parts of the circuit are operated. If bag the doesn't have enough gas in it then it can't push the oil back out with sufficent force. Larger accumulators come with valves and gauges so you can check the pressure and pump them back up if needed, smaller ones may be filled for life.

[distaff_beancounter]

I fly Cougars (GA7), most of which have unfeathering accumulators.

I was under the impression that you cannot check whether they are operating, as part of the pilot's pre flight checks. Only the techies in the hangar, can check them. Well. so far, I have not failed an MEP test, for not checking them. :)

For the MEP test, examiners normally require the pilot to close down 1 engine, at a safe height, in the cruise. The restart should then be done, using the unfeathering accumulators, if fitted.

On the GA7, the restart in flight, (if I can remember without the checklist in front of me :( ) goes as follows:-
Fuel cock on
Mixture to rich
Throttle just open
Both mags on
Prime
Then move prop lever slowly forward, from feathered position, to fully fine

For the GA7, you need a min of about 100k, to windmill the prop to start the engine, therefore, when still on one engine, you may need to lower the nose to get the 100k, just before putting the prop to fully fine.

This usually seems to work for me on MEP tests :)

Presumably if it does not work, and you have to use the starter, then the accumulator may be defective.

[rustle]

distaff_beancounter

I fly Cougars (GA7), most of which have unfeathering accumulators

You fly from Elstree I believe (don't panic, not a stalker - just read it in one of your previous posts :D), and the GA7 "fleet" at Elstree are Cabair ones? Are you sure they have accumulators? Whenever I have borrowed the Cabair GA7s they have not, which is why you need such a high IAS to get the thing spinning again from feathered...

Also why they are so insistent that you don't allow the rpm to drop too much when you do the feathering checks on the ground...

Happy to be wrong, though :)

WCollins

Did a quick Google on unfeathering accumulators - there is quite a lot of info there, including gumf about the nitrogen gizmo.

[bluskis]

Checking the dome pressure is one of the 50 hour/6 month checks under the LAMS schedule for G reg aircraft (that have VP props and domes that is).

[Timothy]

Rustle

The Aztec POH is very insistent that the revs should not drop more than 500 rpm. I can't remember the exact wording, but the implication is "serious engine damage may occur." I can't believe that it is overboost, as the starting point is typically 1500rpm, so I guess that it has to do with vibration.

Distaff

I am starting a new thread on shutting engines down on MEP renewal

W

[rustle]

WCollins

The Aztec POH is very insistent that the revs should not drop more than 500 rpm

Drop more than 500 or drop to 500?

C310 feathering checks, 1700rpm through gate to minimum 1200 rpm.

GA7 feathering checks, 1500rpm through gate to minimum 1000 rpm. (Cabair QRH - don't have a copy of the PoH here)

Both seem to imply that it's a drop by 500...

[Timothy]

Rustle

Yes, drop by 500 rpm.

A drop to 500 rpm wouldn't make much sense as the engine would probably stop, presumably feathered, and the whole point is not to have to restart feathered.

Anyway the Aztec POH is in our downstairs bog, so you can look at it in a quiet moment on Saturday :D

W

[distaff_beancounter]

rustle
Yes, I will have to admit to flying GA7s with Cabair :)
It operates a total of about 12 x GA7s, of which roughly half (I believe?) have factory fitted unfeathering accumulators (AC).

I must disagree, when you say that those without ACs need a higher airspeed to unfeather a prop. In my limited experience (only a PPL & not too good on the techie stuff!) an engine without an AC, will not (& should not) unfeather itself, however much you increase the airspeed. It just sits there fully feathered. I have actually tried this in a GA7, just to see what would happen, & the answer was precisely nothing! :D

In a GA7 with ACs, if you are bumbling along, with 1 engine closed down, at just above blue-line speed of 85K, & do a restart, as you move the prop lever to fully forward, you can actually see the blades twisting from feather to fully fine. BUT, at 85k, the prop will not rotate AT ALL. As you put the nose down & approach 100k, it starts windmilling, & then starts up.

For a GA7 with no ACs, then when using the electric starter motor, after a close down in flight, during the first turn or 2 of the prop, you can see the blades unfeathering.

[bluskis]

AS WC says

Aztec book states 'important to move prop control in and out of feather position very quickly to prevent dropping more than 500 rpm, and causing excessive manifold pressure'.

The normal start point for this check is 1500 rpm, or so I was taught, so min revs are 1000rpm.

I believed the speed of the in and out was to avoid ending up with a feathered prop during pre flight checks, is this so?

[Timothy]

bluskis

Interestingly now I am home I can also look at the POH, where I indeed saw the very sentence you quoted.

Like you, I was also taught to feather between 1500 and 1000, but if you read the pre-flight checklist in the POH it tells you to do it between 2200 and 1700. :confused:

W

[A and C]

On most small twins the accumulator pressure can only be checked with the kit required to charge the accumulator and so is a maintenance function not something that can be checked on a check "A".

[I Fly]

The accumulator pressure is actually pushing against the nitrogen pressure in the dome as well as the spring. So if the nitrogen pressure in the dome has fallen it should be easier to unfeather. Some one is telling you porkies. Perhaps they meant the nitrogen pressure in the accumulator.

[Timothy]

I

The accumulator pressure is actually pushing against the nitrogen pressure in the dome as well as the spring. So if the nitrogen pressure in the dome has fallen it should be easier to unfeather. Some one is telling you porkies. Perhaps they meant the nitrogen pressure in the accumulator.

There are a number of explanations, like the engineer was trying to explain simply, made a slip of the tongue, I misheard, miremembered or misunderstood. The least likely and most offensive suggestion is that he was deliberately lying...apart from the fact that he is honest and reliable, he has nothing to gain and nothing to prove.

W

[bluskis]

Further revision with the Aztec owner handbook gives the following,

Feathering on the ground is unadvisable due to excessive vibration. I've done that and its true.

If the rpm drops below 1000, feathering is inhibited mechanically by a stop pin.

That could be why we were told to use 1500 rpm, and not below 1000rpm.

I don't have those 2200 to1700 rpm figures to hand.

Just a point, not all props have springs, but I don't know how that alters the function of the dome pressure/accumulator pressure.

[rustle]

distaff_beancounter

Apologies for the tardiness - I didn't see your reply :)

I didn't know they (Cabair) had some with and some without - maybe they only hire me the clapped-out ones without because they envisage me doing EFATO or shut-down/restart practise ;)


WRT my comment about higher IAS for non AC-fitted restarts, I think we're at crossed-porpoises (my fault):

I meant that, because the electric starter turns the engine so slowly (and consequently creates little oil pressure), it only just starts to move from fully-feathered - it then can windmill in the faster airflow and this is what drives the oil pump and creates the higher oil pressure to drive the prop towards fine pitch. I didn't mean the higher IAS would windmill a feathered prop.

Whereas one with an AC would move more rapidly toward FP, necessitating a lower IAS (and airflow) to windmill.

But I'm probably completely wrong. DOH! :D

Has that made what I meant clearer or worse?! :confused:

So I guess the biggest benefit of having an AC is where (for whatever reason) you are trying to airstart and don't have a lot of altitude to play with to gain the additional airspeed to make the prop windmill...

[distaff_beancounter]

rustle
Thanks for the reply. I follow your line of thoughts now.
I think that you & I are in agreement. So to sum up:-

A higher IAS WILL help a starter motor unfeather a prop.

A light twin with ACs, gives you a better chance of a successful & quicker restart, than relying on the starter motor only. So, if ACs are fitted, then use them before resorting to the starter. :)

[Tinstaafl]

If accumulators are fitted, you don't have to 'use' them specifically.

Moving the prop. control out of the feather position as part of the normal start does it automatically. That's how all the twins with them that I've flown did it.