A.3.2.4 Emergency power.
AFGS�87219B APPENDIX A
An independent emergency power source shall be provided to supply all essential loads in the event of failure of the primary power source(s).
a. The power rating and total available energy shall be sufficient to supply all loads identified as essential for control of the vehicle and personnel safety.
b. The transfer of essential loads to and from the emergency power source shall be accomplished without transients of a magnitude hazardous to the vehicle or personnel.
c. A convenient means of verifying the operational readiness of the emergency power source prior to flight shall be provided.
d. The emergency power unit should be flight operable over the entire aircraft flight envelope.
e. The emergency power unit should provide power for at least ------------. f. Other emergency power requirements are as follows: ------------.
REQUIREMENT RATIONALE (A.3.2.4)
Despite excess capacity, redundancy, and all other measures taken to ensure a reliable subsystem, total failures of the primary power sources do occur. For multi-generator aircraft there is the tendency to assume that the redundancy of main generators will suffice for the emergency requirement. Experience has shown otherwise. Many modern multi-generator aircraft have experienced total failure of the primary power system. An independent emergency power source is needed for this situation. This requirement has been documented by AFSC Design Handbooks DH 1-6 and DH 2-3.
REQUIREMENT GUIDANCE (A.4.2.3)
a. The power rating of the emergency source must be sufficient to supply the emergency loads identified by an electric load analysis in accordance with MIL-E-7016. As a minimum there should be the capability for safe control of the aircraft. When the emergency source is energy-limited as in the case of a battery, for example, the available energy must be sufficient to ensure operation for the required time period.
b. The response time for initially supplying emergency power is also a consideration. This involves factors such as means of sensing power failure, manual or automatic power transfer, and the time required for the emergency unit to come up to speed or otherwise respond. Should normal power subsequently be restored, manual or automatic means for returning loads to the main source are required. Automatic transfer can be detrimental if it surprises the crew with an additional disturbance at a critical time.
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