B737-300
Q: dual pack tripoff - what is it please?
PACK TRIP OFF Light
(two amber lights on pneumatic control panel - see photo in my previous post)
Illuminated (amber) -
- indicates pack temperature has exceeded limits
- related pack valve automatically closes and mix valves drive full cold
- requires reset.
A
PACK TRIP OFF is when the temperature
inside the air conditioning pack has exceeded a temperature limit.
This usually occurs when the pack is "working hard" - ie trying to reduce the temperature of the air a lot.
A PACK TRIP OFF could happen for a number of reasons - most usually on a very warm day with a cold cabin temperature selected :
--> very warm air into pack + very cold air out of pack = high temperature inside the pack = PACK TRIP OFF <--
It could also happen due to a reduced ram airflow for cooling - eg because the RAM AIR DOORS are not FULL OPEN when they should be FULL OPEN, or because the turbofans are not working when they should be working. A faulty flap position sensing switch could cause these kinds of problems.
The problem with a PACK TRIP OFF is that the affected airconditioning pack is no longer supplying air to pressurise the cabin - the related pack valve automatically closes = no air going into pack so therefore no air coming out of pack. With a single PACK TRIP OFF the other pack can provide sufficient air to keep the aircraft pressurised.
If both packs suffer a PACK TRIP OFF at the same time (a "
dual pack trip off") then there is no airflow coming into the cabin to keep the aircraft pressurised. The pressurisation outflow valve will modulate towards closed to try to maintain pressurisation but all aircraft have small leaks and eventually the aircraft will be depressurised - ie a descent to a lower altitude will be required, maybe even an emergency descent if it is an older aircraft with a high leak rate.
The PACK TRIP OFF non-normal checklist calls for a WARMER temperature to be selected (so the pack is not "working so hard") and pressing the TRIP RESET button (on the pneumatic control panel). If the temperature inside the pack has cooled sufficiently then the PACK TRIP will be cleared and the pack will resume operation. If the temperature inside the pack has not cooled sufficiently then the pack will not reset.
A PACK TRIP OFF does
not mean that air coming out of the pack has exceeded a temperature limit - that would be a DUCT OVERHEAT - see the two DUCT OVERHEAT amber lights on air conditioning panel :
Air conditioning panel (overhead panel)
Pressurisation outflow valve (lower right fuselage below the aft service door)