getting to grips with aircraft performance 如何掌握飞机性能 2(2)

(b)
Every method for establishing minimum flight altitudes must be approved by the authority.”

To assist JAA operators in their choice, guidance material is provided in IEM OPS 1.250, where the most common definitions of published minimum flight altitudes are recalled:
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MOCA (Minimum Obstacle Clearance Altitude) and MORA (Minimum Off-Route Altitude). They correspond to the maximum terrain or obstacle elevation, plus:

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1,000 feet for elevation up to and including 5,000 feet (or 6,000 feet)1.

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2,000 feet for elevation exceeding 5,000 feet (or 6,000 feet) rounded up to the next 100 feet.

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MEA (Minimum safe En route Altitude) and MGA (Minimum safe Grid Altitude). They correspond to the maximum terrain or obstacle elevation, plus:

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1,500 feet for elevation up to and including 5,000 feet.

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2,000 feet for elevation above 5,000 feet and below 10,000 feet.

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10% of the elevation plus 1,000 feet above 10,000 feet.

As a result, the minimum flight altitude above 10,000 feet considered acceptable to carry out studies, is equal to the highest obstacle elevation plus 2,000 feet.
3.1.5. Obstacle Clearance – Cabin Pressurization Failure
A net flight path is not required in the cabin pressurization failure case. The net flight path shall be understood as a safety margin, when there is a risk that the aircraft cannot maintain the expected descent performance (engine failure case).
In case of cabin depressurization, any altitude below the initial flight altitude can be flown without any problem as all engines are running. Therefore, the standard minimum flight altitudes apply and the descent profile must, therefore, clear any obstacle by 2,000 feet (Figure D15).
1 Depends on the method: Jeppesen (5,000 feet) or KSS (6,000 feet)

4. ROUTE STUDY
As a general rule, failures (engine or pressurization) must always be expected to occur at the most critical points of the intended route. Nevertheless, as descent profiles differ, the critical points may differ between the two failure cases. It is important to notice that regulations don’t require to consider performance tocope with both failures simultaneously.
When both failure cases are dealt with separately, the number of critical points and the specific escape routes also increase. As a result, the complexity may engender a supplementary workload for flight crews and a subsequent risk of error.
This is why, whenever it is possible, it must be preferred to define the same critical points and the same escape routes, whatever the failure case. Thus, the reaction time and the risk of mistake are reduced. In such a case, the route study should be based on the most penalizing descent profile (Figure D16).

E. LANDING
1. INTRODUCTION
To dispatch an aircraft, an operator has to verify landing requirements based on airplane certification (JAR 25 / FAR 25) and on operational constraints defined in JAR-OPS and FAR 121. In normal operations, these limitations are not very constraining and, most of the time authorize dispatch at the maximum structural landing weight. This leads to a minimization of the importance of landing checks during dispatch. However, landing performance can be drastically penalized in case of inoperative items, adverse external conditions, or contaminated runways. Flight preparation is, therefore, of utmost importance, to ensure a safe flight.
　
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