B757-200
Maintenance Airframe; Maintenance Avionics; Maintenance Powerplant
Maintenance Technician 20
Communication Breakdown; Confusion
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft; Human Factors; Procedure; MEL
B757-200 aircraft displayed an EICAS status message AFT CARGO FAN. The item was placed on MEL 21-40-2. The Maintenance Dispatch Procedures to comply with the MEL; requires maintenance personnel to open and collar Circuit Breaker (CB) AFT CARGO HEAT FAN located on the P11 panel in position N22. This circuit breaker is actually labeled FAN CONT AFT CARGO. Although this [CB] effectively removes control to the Aft Cargo Fan relay K10084; it also removes 28 volt power from relay K10586 [for the] CARGO SMOKE CONTROL RELAY. If this Maintenance Dispatch Procedure is in effect and the flight crew is required to press the CARGO DEPR switch YQPS1; the V10132 Aft Air Distribution Shutoff valve is deprived of voltage from the C4186 ZONE TEMP CONT VLV CLOSE AFT circuit breaker. This will result in not adequately closing off the main deck Aft Distribution Manifold. Since package freighters have Class E cargo compartments; they rely solely on oxygen starvation in the event of a fire. RB-211 Engines. Heating System.
Reporter stated the response from their Fleet Manager was excellent and the Manager immediately cancelled the application of their MEL Maintenance Procedure to open and collar the Circuit Breaker (CB) for the AFT CARGO HEAT FAN after understanding Reporter's concerns. The AFT CARGO FAN was applicable to the upper Main Deck Aft Zone area on their Boeing factory built B757; all cargo aircraft. No Lower Lobe Aft Cargo Fan or resistance type heater exists on factory built B757 freighters; only on passenger and passenger/cargo combi aircraft.Reporter stated Boeing had added additional circuits on factory built freighters to the AFT CARGO HEAT FAN CB that was actually labeled FAN CONT AFT CARGO without actually identifying as such; except on the wiring schematics. Boeing understood his concerns and along with his Engineering department their company MEL was revised to have the actual FAN CONTROL CB pulled and collared; not at the cockpit Overhead CB panel but in the lower E&E Compartment. That CB directly controls the AFT CARGO FAN which is similar to a recirculation fan but no heat.Reporter stated that electrical power to the CARGO DEPR switch; also known as the 'panic button'; came from the same FAN CONT AFT CARGO CB at the cockpit Overhead panel. The Cargo DEPR switch does not depressurize the aircraft's cabin during a Fire/Smoke Warning; but rather shuts off one Air Conditioning Pack; preferably the Right Pack (assuming a two pack operation was functioning) and reduces the Left Pack to a Low flow status. Pulling the FAN CONT AFT CARGO CB at the Overhead panel would shut-off power to the CARGO DEPR switch and not close the Aft Shut-off valve that supplies air to the upper deck AFT Cargo area. Their B757-200 packaged freighter Cargo Compartments do not have a Fire Suppressant System. Closing off air to the Main Deck Cargo area is the only option. He works in Technical Support and has twenty years Line Maintenance experience.
An Aircraft Maintenance Technician (AMT) describes the potentially unsafe condition that would exists if a Fire/Smoke Warning occurred in the Main Cargo Deck areas of factory built all cargo B757-200 aircraft and MEL 21-40-2 had been applied. The FAN CONT AFT CARGO Circuit Breaker (CB) would be opened and collared at the cockpit overhead panel and the aft air distribution shut-off valve would not close. No Fire Suppressant System exists in Class 'E' Cargo compartments on B757s.