GM1 MMEL.145(d) Justification of MMEL items

ED Decision 2014/004/R

QUANTITATIVE SAFETY ASSESSMENT

(a)     Items for which a quantitative safety assessment is carried out to supplement the qualitative MMEL development process in accordance with CS MMEL.145(d) should be reported.

(b)     Items for which the probabilities per flight hour of 1.10^-8 for Catastrophic failure conditions and 1.10^-6 for Hazardous failure conditions are not met in that dispatch configuration should be reviewed with the Agency. The following guidance applies to these proposed dispatches. This guidance includes equations to control how long these configurations are allowed to exist, such that the fleet average objectives will be achieved.

(1)     For catastrophic failure conditions:

(i)      A probability per flight hour under dispatch condition of ≤ 1.10^-8 is the objective when dispatching with the inoperative item. When this objective is met, no calculation for a maximum allowable dispatch time is considered necessary.

(ii)     A limited number of items may be considered when 1.10^-8/FH objective is not met. In these cases, the maximum allowable probability per flight hour when dispatching with the inoperative item should not exceed 1.10^-7/FH, and the maximum dispatch time should be less than that calculated using the following Equation (1).

(iii)     The 1.10^-8/FH objective and 1.10^-7/FH upper limit apply to each catastrophic failure condition involving the inoperative-at-dispatch MMEL item. If more than one top level event is involved, the maximum allowable dispatch time should be the smallest of those calculated for the affected failure conditions.

Equation (1):

Where

Max_Disp_Time_CAT[FH] = Max Dispatch Time [flight hours]

PF [1/FH] = Probability of failure condition [per flight hour] under dispatch condition

FR [1/FH] = Failure Rate of proposed MMEL item [per flight hour]

(2)     For hazardous failure conditions:

(i)      A probability per flight hour of ≤ 1.10^-6 is the objective when dispatching with the inoperative item. When this objective is met, no calculation for a maximum allowable dispatch time is considered necessary.

(ii)     A limited number of items may be considered when 1.10^-6/FH objective is not met. In these cases, the maximum allowable probability per flight hour when dispatching with the inoperative item should not exceed 1.10^-5/FH, and the maximum dispatch time should be less than that calculated using the following Equation (2).

(iii)     The 1.10^-6/FH objective and 1.10^-5/FH upper limit apply to each hazardous failure condition involving the inoperative-at-dispatch MMEL item. If more than one top level event is involved, the maximum allowable dispatch time should be the smallest of those calculated for the affected failure conditions.

Equation (2):

Where

Max_Disp_Time_HAZ[FH] = Max Dispatch Time [flight hours]

PF [1/FH] = Probability of failure condition [per flight hour] under dispatch condition

FR [1/FH] = Failure Probability of proposed MMEL item [per flight hour]

Note 1  The two equations given above for maximum dispatch times for MMEL items or functions involved in catastrophic or hazardous failure conditions provide dispatch times that are compatible with the fleet average top level reliability requirements of CS 25.1309(b).

Note 2  Equation (1) would yield a maximum operating time in the particular configuration to be ≤ 1 % of the fleet operating time when the MMEL dispatch configuration has a probability of 1.10-7/FH.

Note 3  Maximum dispatch times, as calculated using the above equations or other appropriate methods, should be maintained by the applicant’s operations/MMEL group. That group will work with the Operations Evaluation Boards (OEBs) to decide on an acceptable MMEL entry.

Note 4  Probabilities used in above paragraph are average probabilities per flight hours as defined in AMC 25.1309.

(3)     Dispatch times should primarily be based on operational considerations. Whenever possible, the MMEL entry should use the standard Rectification Interval Categories by rounding the calculated maximum dispatch time (in flight hours) to a conservative Category (based on maximum aircraft utilisation per day), not exceeding the C category.