B737 Undifferentiated or Other Model
Maintenance Repairman
Maintenance Technician 22
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Aircraft
I would like to inform you that there is a system error in the precooler control system in the engine bleed system on Boeing 737 aircraft.The precooler control valve used in this system; when 390 F sensor remains open or if there is air leakage from the 390 F sensor control pressure sense line or wing thermal anti ice solenoid remains open; precooler control valve goes fully open with spring loaded. If Precooler control valve full open or hold any position; there is MEL restriction for icing condition As far as I know icing condition is a restriction required for wing anti ice.In the above mentioned cases (precooler control valve goes to full open position) engine bleed system does not give any malfunction or warning. This is a situation that conflicts with MEL item.An important restriction; such as icing condition; is that the system does not give a fault or warning under the conditions I have described above.However; many Boeing 737 aircraft may be flying in this condition. Any operator cannot be aware of this situation because no malfunction is seen. This situation create a dangerous situation; especially in winter.
Reporter stated that the precooler control valve on the B737 is poorly designed and subject to failures.