Hi,

As you know sometimes hydraulic power unit (mule) is used for aircraft maintenance especially inside hangar. I have been told there is portable external oil cooler (PEOC) which is used inside hanger on 737 MAX. This PEOC is used to cool the hydraulic pumps (electrically driven) when the fuel tanks are empty inside hanger for maintenance. This PEOC is used kind of alternate way of mule but with the use aircraft hydraulic pumps (electrically driven).

a) Is this oil cooler (PEOC) used a lot for airlines and MROs (maintenance, repair, and overhaul)

b) Is this oil cooler (PEOC) used only for 737 MAX or all Boeing aircraft?

c) Is this oil cooler (PEOC) used also for Airbus aircraft or other aircraft?

Feedback appreciated.

aerotech777 wrote:

a) Is this oil cooler (PEOC) used a lot for airlines and MROs (maintenance, repair, and overhaul)?

I've never personally seen such a device used.

aerotech777 wrote:

b) Is this oil cooler (PEOC) used only for 737 MAX or all Boeing aircraft?

I presume such a device would be needed for all Boeings if you were running electrically driven hydraulic pumps for any appreciable amount of time with empty fuel tanks.

aerotech777 wrote:

c) Is this oil cooler (PEOC) used also for Airbus aircraft or other aircraft?

I presume Airbus' would also need some way to cool the hydraulic fluid if the usual method is not available.

Regards, JetMech

You must be referring to an External Hydraulic Cart. It is used in heavy Maintenance eg when the Aircrafts own Hydraulic pumps are not available for maintenance.
They are a source of hydraulic pressurisation.

Thanks for the feedback.

I've never personally seen such a device used.

It seems this device is relatively new, may be used since 2 years. That's why I am asking if AMTs are using this device.

You must be referring to an External Hydraulic Cart. It is used in heavy Maintenance eg when the Aircrafts own Hydraulic pumps are not available for maintenance.
They are a source of hydraulic pressurisation.

As I said this is not Hydraulic Power Unit (mule) or what you called External Hydraulic Cart. This device is an External Oil Cooler which is used to cool the aircraft hydraulic pumps when the fuel tanks are empty
or not enough fuel to cover the heat exchanger. As you know in such cases AMTs can't use the hydraulic pumps more than 2 minutes. So it seems with this equipment, it is possible to use the hydraulic pumps continually.

I have been told that this device is mentioned on 737 MAX AMM (aircraft maintenance manual). May be mentioned in certain AMM not all. If I am not mistaken AMM are customized.

aerotech777 wrote:

a) Is this oil cooler (PEOC) used a lot for airlines and MROs (maintenance, repair, and overhaul)

No, at least never heard of such device at where I work.

aerotech777 wrote:

b) Is this oil cooler (PEOC) used only for 737 MAX or all Boeing aircraft?

Never touched the MAX and don't have the AMM on hand for the MAX, but for all other Boeing aircraft I have never seen or heard of the use of such device.

There is only mention in the AMM to avoid operating the hydraulic system without enough fuel in tank for cooling, and to monitor hydraulic temeprature in case of overheat. But generally speaking at maintenence stage there is no real reason to operate the hydraulic system for an extended period to get to the point of having overheating hydraulic. If you do need to operate for long time you should be at the latter stage of maintenece and really should be thinking of refuelling the aircraft to test out the APU, pneumatic system, AC Pack and so on.

Also a big thing is if there is no quick disconnect designed into the existing hydraulic system to hook up to such device, as per maintenence manual (no AMM say you can use it, you can't use it), then it's a big NO - you don't want to disturb any connection without a really strong reason. I would rather put fuel into the tank rather than disturbing the system and having to do a leak check afterwards. And if the aircraft is powered up the fuel tank should be closed up and ready to take on fuel.

aerotech777 wrote:

c) Is this oil cooler (PEOC) used also for Airbus aircraft or other aircraft?

Not much experience on Airbus, but I guess it's the same answer.

Thanks for the feedback.

There is only mention in the AMM to avoid operating the hydraulic system without enough fuel in tank for cooling, and to monitor hydraulic temeprature in case of overheat. But generally speaking at maintenence stage there is no real reason to operate the hydraulic system for an extended period to get to the point of having overheating hydraulic. If you do need to operate for long time you should be at the latter stage of maintenece and really should be thinking of refuelling the aircraft to test out the APU, pneumatic system, AC Pack and so on.

If I am not mistaken, the AMM mentions to not operate the hydraulic pump(s) more than 2 minutes if there the fuel tank quantity is less certain quantity (at least on 737). I assume the fuel tanks are empty inside the maintenance hangar for safety reason. Hence the oil cooler (PEOC) can be useful for the maintenance. You can do some tests that need the hydraulic system inside the hangar, instead of waiting until the plane is moved outside the hangar for example in D or C check (save time). With the use of hydraulic power unit (mule), you can't test certain components (for example case drain filter of the electric pump) of the hydraulic systems upstream the pressure hydraulic module because of the check valves in this module.

Also a big thing is if there is no quick disconnect designed into the existing hydraulic system to hook up to such device, as per maintenence manual (no AMM say you can use it, you can't use it), then it's a big NO - you don't want to disturb any connection without a really strong reason. I would rather put fuel into the tank rather than disturbing the system and having to do a leak check afterwards. And if the aircraft is powered up the fuel tank should be closed up and ready to take on fuel.

In 737 there is a quick disconnect on the electric pumps. I have been told the PEOC is designed so you can use the quick disconnect fitting, so the AMT will not to disturb any connection/line. No need to do any leak check on the hydraulic system after the use of the PEOC.

I think the key to this is understanding which aircraft use a hydraulic to fuel heat exchanger.

I have used many external hydraulic kidneys especially on earthmoving equipment to drive the hydraulic system, we had a couple of inline filters to clean the metal out of the system.

aerotech777 wrote:

If I am not mistaken, the AMM mentions to not operate the hydraulic pump(s) more than 2 minutes if there the fuel tank quantity is less certain quantity (at least on 737). I assume the fuel tanks are empty inside the maintenance hangar for safety reason. Hence the oil cooler (PEOC) can be useful for the maintenance. You can do some tests that need the hydraulic system inside the hangar, instead of waiting until the plane is moved outside the hangar for example in D or C check (save time). With the use of hydraulic power unit (mule), you can't test certain components (for example case drain filter of the electric pump) of the hydraulic systems upstream the pressure hydraulic module because of the check valves in this module.

We do defuel if there is a need for jacking up the aircraft and perform structural work (reduce structural stress), fuel tank inspection (SFAR88) or sometimes simply doing a sump drain (water). Otherwise for short maintenence turnaround (no definite line here but somewhere like a 14 day check) it's not unusual to see fuel being left in the tank for the maintenence duration. Fuel being dumped out are considered as contaminted fuel so it's not a decision taken lightly.

Inside hangar with the electric pumps pressurising the hydraulic system the temperature rise is actually slower than you would expect. On a 767 unless it's a really hot day outside you could get away running all the flight control checks without any significant hydraulic temperature rise. For large C/D checks the maintenence duration is long enough that hooking up a hydraulic fluid cooler won't really save you much time to justify its usage.

aerotech777 wrote:

In 737 there is a quick disconnect on the electric pumps. I have been told the PEOC is designed so you can use the quick disconnect fitting, so the AMT will not to disturb any connection/line. No need to do any leak check on the hydraulic system after the use of the PEOC.

If it's something only available to the 737Max then problably it won't be high on the shopping list, unless if your MRO has a committed contract for at least a few dozens which would justify the cost of this unit.

zeke wrote:

I think the key to this is understanding which aircraft use a hydraulic to fuel heat exchanger.

I only just realised it was the case drain flow by itself that gets cooled. I always thought it was the entire return flow prior to the reservoir.

aerotech777 wrote:

If I am not mistaken, the AMM mentions to not operate the hydraulic pump(s) more than 2 minutes if there the fuel tank quantity is less certain quantity (at least on 737).

A or B system EDP or EMDP cannot be run for more than 2 minutes with less than 760kg of fuel in Main 1 or 2 respectively (737-800).

Regards, JetMech

My companies version of the Boeing 737 MAX and NG AMM includes the PEOC. Unfortunately it references using instructions on the PEOC to connect it to the plane.

My companies version of the Boeing 737 MAX and NG AMM includes the PEOC. Unfortunately it references using instructions on the PEOC to connect it to the plane.

Thanks for confirming that the PEOC is mentioned in the 737 AMM.

I only just realized it was the case drain flow by itself that gets cooled. I always thought it was the entire return flow prior to the reservoir.

The case drain flow cools and lubricates the pump.That's why the overheat sensor is located case drain line. But when using the PEOC, it seems the entire return flow circulates through the PEOC for the cooling. This way the case drain is not disconnected from the heat exchanger.

Since I mentioned the overheat sensor in the case drain line, I am wondering why the 737 EDP (engine driven pump) case drain line was not fitted with overheat sensor even the 737 MAX. I seems all Boeing and Airbus aircraft were fitted with EDP case drain overheat sensor except may be the old Boeing aircraft (707, 727, 737). I might be wrong for saying such statement.

We do defuel if there is a need for jacking up the aircraft and perform structural work (reduce structural stress), fuel tank inspection (SFAR88) or sometimes simply doing a sump drain (water). Otherwise for short maintenence turnaround (no definite line here but somewhere like a 14 day check) it's not unusual to see fuel being left in the tank for the maintenence duration. Fuel being dumped out are considered as contaminted fuel so it's not a decision taken lightly.

As you know there are regulations in order to move aircraft inside the hangar and perform maintenance. I assume the regulations are not the same depending if the aircraft is defueled or no. For example the electric system is de-energized and there are some restrictions of the use of certain equipment inside the hangar.
What are the restrictions or the regulations for performing aircraft maintenance with the fuel left in the tanks inside the hangar?

We never defuel our aircraft before maintenance unless it is absolutely necessary. Defueling is very expensive. Not only do you lose the fuel you already paid for, you have to pay for its disposal. Once fuel enters an aircraft it cannot be resold. It could be used to refuel other aircraft of your own fleet but you want to avoid that to prevent cross-contamination with microbes.
When there is work to be done in a fuel tank we transfer the fuel from that tank to another tank and drain only what the pumps can't reach.
When we know a certain amount of fuel is needed for testing during or after the maintenance event, we refuel the aircraft to that amount beforehand. This might be a couple dozen tons for example when a high power engine test run is required.