Gazelle Astazou oil system

[ronnie barker]

Hi i am trying to find out what the purpose exactly is of the mag probe in the oil system. I know it is there to collect debris but where from? the book are not very clear one part say it is there to protect the scavenge pump and another part says its there to protect the oil cooler from blockage.

a little clarification would be good

nmany thanks

[ShyTorque]

Ronnie B,

It's 24 years since I instructed on the RAF's Gazelle HT2 (how time flies), but having looked at my copy of the technical notes, which include a schematic diagram, I think the real answer is "neither".

The mag plug is situated after the scavenge pumps, so it cannot protect them.

The only thing that would efficiently 'protect' either the scavenge pumps (there are three of them, btw, not one) or the cooler, is a filter. There is only one filter, in the system, after the pressure pump.

A mag probe may allow some debris to get past, especially if it were non-ferrous.

I think the answer is this:

On the Gazelle's Astazou engine, the single mag probe is situated in the only common oil line after the scavenge pumps, so any metal from either of them should be caught by it for the engineers to find. If it were placed elsewhere it would need others to complement it.

[Rigga]

Generally:
Mag Plugs are an optional modification that can be fitted to any oil system, on any aircraft. Expensive one's have electric currents that, when closed by the debris on the Plug, can give a cockpit indication. Some posh ones can even send a current to the plug to "burn off" the debris. (known as Fuzz-Burners)

Thay are normally fitted before a filter because the filter catches most of the debris!

Mag Plugs are not there to clean anything. They are there purely as a means of indication for the presence of metallic substances moving around in the oil system. As ST says they won't pick up alluminium particles or rubber seals.

The majority of wear/harm to your engine/gearbox is caused by the ingestion of ferrous metal often originating from within the assembly itself.

Every Mag Plug fitted to your aircraft should be included in your maintenance programme and have periodic inspections. You would do well to keep to those inspections. Such a cheap mod will earn its keep.

If you find anything on one - DONT PULL IT OFF! You should get ALL found debris properly assessed by an expert. At first you may feel this is an unnecessary cost, but it will save you money in the long term. (and if you cant afford it you shouldn't own the aircraft!)


The metallic substance gathered by it may be:
1. Part of the 'build debris' that remains in the system or, more importantly,
2. It may indicate an item beginning to wear or breakdown.

It may show chrome or bright silver coloured particles that may be the finish from bearings or gears that may be classified as "negligible". Sometimes you may see what is described as a "Christmas Tree" - a blob of silver particles all the way up the magnet - Even this (worrying as it looks) MAY be regarded as negligible (by your expert) - depending on what it is and when you get it! It MAY be build debris - It may NOT!

In my experience, the time to worry is when the particles are dull grey or gold.

Hope this helps

[helimarshaller]

Similar to Shytorque, I taught Gazelle & Astazou to REME Aircraft Technicians at Middle Wallop back in 1985. What has already been said is good info.

As the probe is in the return line from the oil pump (scavenge) to the oil cooler it will collect debris from the accessory gearbox, centrifugal compressor bearings, & the rear bearings.

Where the debris has come from can only be identified by someone who had been trained to know what to look for. The Army also had Experts at Middle Wallop and Detmold (long gone) who could decide what action needed to be taken.

Importantly it should be noted that if the engine Mag Probe is left out when the engine is running, the oil flow to the oil cooler is shut off and therefore no oil gets to the pressure pump to be passed around the system.

Hope this helps.

Steve