Norbert Pohl

Request to add an additional method acceptable for compliance with the corresponding requirements of this AD. Lufthansa notes that in paragraph (f) and g(1)/g(2) actions done in accordance with Boeing Alert Service Bulletin 737-21A1156 are acceptable for compliance. These actions prevent loss of all attitude indications from both the standby indicator and EFIS displays in case of battery bus failure. The standby indicator is powered by the battery bus, just as the Normal Supply Fan used for EFIS cooling. However, if the standby bus fails, the Equipment Cooling Supply Off Light does not show any indication, assumed that the Equipment Cooling Supply Switch is still set to Normal. The EFIS displays overheat and consequently fail. Lufthansa would like to have the FAA consider that another method to face this unsafe condition is to append an Abnormal Procedure Task to the Flight Crew Operations Manual. If the battery bus fails the battery bus DC Voltmeter indicates 0, and both N1 indicators are not powered, making it possible to realize and identify a battery bus failure. If the Equipment Cooling Supply Switch is set to Alternate, a safe landing can be permitted if done at the nearest suitable airport. Shifting the Equipment Cooling Supply Switch to Alternate will allow the Alternate Supply Fan to still cool the EFIS displays and prevent them from failure and subsequent loss of attitude indication. Lufthansa suggests accepting this Abnormal Procedure in case of battery bus failure as an alternate method of compliance to this AD.