From Skybrary In commercial operations, it is highly desirable that the most direct route between two airports be flown whenever possible. Where that route involves the overflight of extensive areas of high terrain, it is critical that escape routes and procedures be developed and used in the event that an emergency requires that the aircraft must descend to an altitude that is below the Minimum Obstacle Clearance Altitude (MOCA) (MOCA). In many parts of the world, aircraft are routinely flown over terrain that has minimum obstacle clearance altitudes (MOCA) exceeding 10,000'. In most areas, however, the relatively short exposure time to the high terrain negates the requirement for predetermined escape routes and procedures. There are several exceptions to the premise of minimum exposure time. These exceptions include central Asia due to its very extensive areas of high terrain. Avoidance of these areas by transiting aircraft could potentially add hundreds of extra miles to a given route and result in a substantial increase in flight time and the associated costs. This is not desirable from a commercial standpoint. To satisfy the commercial imperative while maintaining an acceptable level of safety, operators have developed escape routes and the associated procedures for use in the event of an emergency whilst overflying extensive high terrain. The primary threats to safe flight over extensive areas of high terrain are those situations which result in the immediate requirement to initiate a descent. These threats include: Engine failure Loss of pressurisation Fire Analysis of these threats against the capabilities of the specific aircraft type and configuration will determine which of them defines the most restrictive terrain clearance profile. This, in turn, will determine what (if any) limitations must be applied to any route of flight that might be under consideration. An engine failure or an emergency, which requires the immediate shutdown of an engine, will normally result in the requirement for a descent. If the one engine inoperative ceiling for the anticipated weight, corrected as required for the existing conditions, exceeds the maximum terrain height, the route is not limited by engine out performance. If, on the other hand, the aircraft is not able to maintain level flight at an altitude at or above the MOCA with one engine inoperative, the maximum exposure to the high ground must be limited by the distance that the aircraft could fly, using a drift down profile, prior to descending below the minimum safe altitude. In the event of loss of pressurisation, the standard procedure is to initiate an emergency descent to the higher of 10,000' or the Minimum En-route Altitude (MEA) (MEA). If the MEA, as corrected for existing conditions, is above 14,000' (13,000' for some National Aviation Authorities (NAA)), continuing the descent to MOCA would be prudent. If the MOCA is also above 14,000', the route of flight will be limited by the availability of supplemental/emergency oxygen supplies. Flight crew supplemental oxygen is rarely limiting; however, passenger emergency oxygen, when provided by Chemical Oxygen Generators, is only available for a limited amount of time. This time is dependent upon the capacity of the generators that have been installed in the aircraft concerned. Regulations require a minimum passenger oxygen supply of 10 minutes. The majority of chemical generators have a useful life of between 12 and 20 minutes depending upon the type. For flight over extensive areas of high terrain, the planned route must allow that an emergency descent to 14,000' (13,000' for some NAA) or lower can be safely made prior to exhaustion of the passenger oxygen generators. This descent will occur while following a pre-planned escape route that must also allow further descent to below 10,000' within 30 minutes of emergency oxygen supply exhaustion. In these circumstances, the descent will be progressive, based on the safe al