Pitot / static head leak rate??

[camlobe]

Hi folks.
Wonder if any old-school leckies can help me with this one. Looking after an ex-RAF Bulldog amongst other things. Tried to do a leak check of the pitot system. From pipes at the head to the ASI - perfect. Connect the head (Avimo Mk 8D), blank off the drain hole and try again - leaks out faster than I can pump up. If I do a leak check on the head itself on the bench after blanking the pipe offtakes and taping up the static slots, all OK. Take the tape off the static slots and it just dumps the pressure, so I have a massive leakage across the head from pitot to static. Have been told that there is an 'allowable leak rate' on these heads between the pitot and the static. Said leak rate is quoted as 5 kts in three minutes. Problem I have with this quoted rate is; a) I seem to remember the allowable total system leak rate was 5 kts in three minutes, and b) it is an ex-Eng Off advising me of this leak rate, so pardon my sceptisism.

Your help, wisdom and experience is required.

Many thanks in advance.

camlobe

 

[Wonky Tonka]

Sounds about right to me.

Most systems are allowed a leak rate of about 5kts / 3mins.
That's usually on the whole system.

Sounds like you have quite a serious hole somewhere. Best get some Snoop on the pipes DT_Xtremez_31:

Could be that the pressure head seals are shot to buggery though......especially if you are using an old piece of kit.

Even the Gucci (?) kit we use on the Tonka is prone to leaking like a barsteward if the seals aren't set up properly.

 

[Lamptramp]

Hi folks.
Wonder if any old-school leckies can help me with this one. Looking after an ex-RAF Bulldog amongst other things. Tried to do a leak check of the pitot system. From pipes at the head to the ASI - perfect. Connect the head (Avimo Mk 8D), blank off the drain hole and try again - leaks out faster than I can pump up. If I do a leak check on the head itself on the bench after blanking the pipe offtakes and taping up the static slots, all OK. Take the tape off the static slots and it just dumps the pressure, so I have a massive leakage across the head from pitot to static. Have been told that there is an 'allowable leak rate' on these heads between the pitot and the static. Said leak rate is quoted as 5 kts in three minutes. Problem I have with this quoted rate is; a) I seem to remember the allowable total system leak rate was 5 kts in three minutes, and b) it is an ex-Eng Off advising me of this leak rate, so pardon my sceptisism.

Your help, wisdom and experience is required.

Many thanks in advance.

camlobe

Pump up to 130 knots leak max 5 knots in 3 minutes.

Is the ASI indicating when you pump? The leak you describe sounds like it is disconnected.

Check the tester by putting you thumb over the end of the pipe and pump - the risk is that your thumb might leak.

Also check that there is only one drain hole, and it is properly sealed.

 

[camlobe]

Wonky Tonka and Lamptramp,

Thanks for your rapid responses. Regarding the 5 kts in 3 mins, glad I haven't lost all marbles, even if the snow is on the roof.

'...old piece of kit'?? Me or the aircraft? The leak tester I am using is the good 'ole Negretti puffer box, freshly serviced. I use a Smiths 145BTE test set for calibration of systems.

If I leak test the system without the head, it is perfect. No loss of pressure in half an hour. System and tester satis.

And I can confirm just one drain hole in the head.

Regarding head seals, how difficult is it to dismantle an Avimo head to access the seals? (rules are different in the civvy world) or are you referring to the seals on the jonny adaptor?

camlobe

 

[Wonky Tonka]

Initially, check the seals on the adaptor

They might just need reseating (the ones for the MCADTS on Tonka were always getting dislodged).

Wouldn't know about the leagalities of opening up the pressure controller though.

 

[MingMong]

Pump up to 130 knots leak max 5 knots in 3 minutes.

This I have found varies from aircraft to aircraft. Althought the 5 knots in 3 seems to be the same on all the types I have worked on, the pressure that you pump the system up to has varied.

If it is an old Bulldog that you are working on, I would suggest that you check the correct figures for that aircraft, i'm not sure how 'somebody on e-goat told me' would stand in an investigation.

 

[sarboy22b]

Could be a number of things,could easily be the seals on the pitot adaptor-they seem to be a pain on most a/c Sea Kings are complete barstewards, you dont have any drain traps disconnected do you? also as Lamptramp said make sure asi connected correctly and is indicating as you pump up the system. As people have said 5kts in 3mins is a usual leak rate for a complete system. Have you had the system apart by the way and if so did you leak test it before you disturbed it-unlike when fairies do it!DT_Xtremez_28:

 

[camlobe]

Update

Update

Thank you all for your valued inputs. It has been a big help.

Things seem to have developed in a small way. The aircraft owner has 'obtained' a couple more pitot heads. So I subjected these heads to a leak check. One, brand new ex-mil stock, dumps the pressure faster than a blow-off valve. The other is as I would have expected, displaying a very slight (read calibrated) leak across the head, approx 10 kts per minute. Only difference between the two is, the dumper is a Mk 8D, and the 'better' one is a Mk 8C. According to the parts catalogue, the Mk 8D is a post Mod 124 fit, the Mk 8C is pre-Mod 124.

Anybody here able to expand on the Mod as I don't have a copy of the Vol 2.

camlobe