ATA 21

Bleed Line Over-pressurization
Your flight crew has returned from a routine flight and reported the illumination of the RED BLEEDLINE OVERPRESS warning light during the landing roll out. You perform an operational check of the bleed air system per the Aircraft Maintenance Manual. You are unable to duplicate the discrepancy. All functions appear normal and no abnormal indications are observed during the checks.

Questions:

1. What pneumatic system device(s) activate the BLEED LINE OVERPRESS warning light?
2. What bleed air control configuration may cause this condition (over-pressure/temperature in the bleed manifold) to occur during landing operations?
3. Why did the warning appear, and was it a true or false indication?

Answers:

1. The BLEED LINE OVERPRESS warning light is activated by either the over-temperature or over-pressure switch located on the bleed air common manifold upstream of the refrigeration unit.
2. If the cabin air selector is set to “RIGHT”, the ground bypass air valve will open upon landing because all conditions are met. When the crew deploys reversers and increases engine RPM with the piggyback lever(throttles are at idle), pressure and temperature in the bleed air common duct may rise above over-pressure switch or over-temperature sensor limits. As the right engine spools up, the open ground bypass air valve routes unregulated HP bleed air directly to the common manifold.
3. The BLEED LINE OVERPRESS warning will appear and it will be a “true” indication.