B737-800 question about mmel 21-31

[DongJoon Choi]

Hi there,

I would like know about relation between recirculation fans and packs

Question:

The point(my question) is that why left pack must operate and Pressurized flight must be required if either recirculation fan (left or right) is inoperative, and why OAT affects the operation.

Please respond to my question and tell me the philosophy about that.



Please respond to my question.

MEL say :

Left fan may be inoperative provided the left pack
is operating when OAT is above 100 degrees F (38
degrees C).

Right fan may be inoperative provided:
a) The left pack is operating when OAT is above
100 degrees F (38 degrees C), and
b) Flight is conducted pressurized.

both fans may be inoperative provided:
a) OAT remains below 100 degrees F (38
degrees C), and
b) Flight is conducted pressurized.

[rmm]

From memory the L/H pack feeds the cockpit.

The OAT above 38'c would only be encountered on the ground and low altitudes. So, if you were at the gate on a 38'c + day with the heat generated by the cockpit instrumentation the temp in the cockpit would probably exceed 50'c without the pack running. You probably wouldn't perform well in this situation.

If you were unable to preform a pressurized flight it would suggest another related system is U/S, so why would you want to go?

The other issue is how the recirc fans and packs are powered. I don't the electrical schematic but, say you lose an engine generator, how would that effect the system?

[Kuchan]

The recirculation fan circulates air ( exhausted air from the main cabin and electrical equipment bay) to the main cabin at a rate of a full cabin air exchange every 2 or 3 hours. Its purpose is to reduce the air conditioning pack load and the engine bleed air demand.

The regulation is to have full cabin exchange every 2 or 3 minutes. Only pressurization with engine running can achieve it.

Pressurization means with pack on. U/S Pack one is "no go" in most cases as Pilots are essential to LIVE AND NOT LET DIE and SUFFER above 38 Degrees C . Right pack is go under MEL.

[cone zone]

I might be wrong but I wouldn't mind betting that it is to protect the avionics more so than the pilots.

[Kuchan]

B737 NG and A320 use almost the same system of having their own fans ( supplying and exhaust) to cool avionic equiptments.

A330 uses recirculating air to cool avionic equipments.