PACK FAIL (amber) due to inlet overheat possible causes:

• Fan Air Valve (FAV) (Bleed Air System) failed to open fully while PIT was above 232 Deg. C for 30 sec, or above 260 Deg C for 10 sec. This condition should trigger the associated FAV to fail to open in the CAIMS Flight Faults or NVM.  A quick way to check the FAV is to verify that it shows full open (on CAIMS raw data) when APU bleed is running.
• The pack flow is too high for the precooler cooling capabilities. If the PIFS or PIPS used to calculate the Pack Flow have drifted (without being out-of-range) the resulting true flow may be too high for the precooler to ensure proper cooling.

Note that if PACK FAIL occurs right after the bleed is transitioning from LP to HP (as the auto-throttles are moving back), it could be due to an FCV issue. In this case you will also have a PACK FAULT (cyan) CAS message and the NVM will report an FCV Failed Open.

Unable to Maintain Commanded Temperature in Cockpit and Forward Cabin

The Pack Discharge Pressure Connection (see photo) to the Air Cycle Machine is where the pack discharge pressure is sensed for comparison with cabin pressure by the Pack Discharge Pressure Sensor (PDPS) which is a differential pressure transducer located under the baggage floor board.

In order for the Pack Discharge Pressure Connection to sense pack discharge pressure it must pass through the anti-ice chamber which is an integral part of the Air Cycle Machine. If the fails, or if the PDPS sense line has a cracked flare, this sets up a runaway condition since the PDPS is now sensing the bleed air pressure from the anti-icing system as well as pack discharge pressure.

If the ACSC senses that the pack discharge pressure is high relative to cabin pressure, it will initiate an anti-ice cycle and open the temp control valve and the affected pack will runaway hot.

Lessons Learned:

It is not uncommon to see Pack Discharge Temp splits on the Air Conditioning Synoptic page of up to about forty degrees. If temperature splits in excess of forty degrees are observed, a problem may be present. However, there will not be any operational impact until the split reaches about seventy degrees since a healthy Pack has enough excess capacity to compensate for a degraded pack until this point.

It has been shown that if the Air Cycle Machine outlet duct is loose, it compromises the Pack Discharge Pressure Connection o-ring seal.