Overview
In commercial aviation, flying the shortest route between two airports is ideal. However, when this route crosses extensive high terrain, it’s crucial to have established escape routes and procedures. These are necessary in emergencies requiring the aircraft to descend below the Minimum Obstacle Clearance Altitude (MOCA).
General
Around the world, many aircraft fly over terrain where MOCA exceeds 10,000 feet. In most cases, the short exposure to high terrain eliminates the need for predetermined escape routes. Yet, regions like central Asia, with vast high terrain, require special consideration. Avoiding these areas can significantly extend flight distance and time, which is not favorable for commercial operations. To balance safety with efficiency, operators have created escape routes for emergencies over these challenging terrains.
Threats
The main threats to safe flight over high terrain include situations that necessitate an immediate descent, such as:
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Engine failure
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Loss of pressurization
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Fire
Analyzing these threats against specific aircraft capabilities helps identify the most restrictive terrain clearance profile, influencing route limitations.
Engine Failure
An engine failure typically requires a descent. If the aircraft’s one-engine-inoperative ceiling exceeds the maximum terrain height, there are no restrictions based on engine performance. Conversely, if level flight cannot be maintained above MOCA, the maximum exposure to high terrain must be limited based on how far the aircraft can drift down before descending to a safe altitude.
Loss of Pressurization
When pressurization is lost, the standard procedure is to initiate an emergency descent to the higher of 10,000 feet or the Minimum En-route Altitude (MEA). If the MEA exceeds 14,000 feet, continuing the descent to MOCA becomes advisable. The availability of supplemental oxygen for passengers is crucial since it typically lasts only 10 to 20 minutes, depending on the aircraft’s chemical oxygen generators. Therefore, planned routes must allow for a safe emergency descent to 14,000 feet or lower before oxygen supplies run out.
Fire
While a fire does not directly limit altitude, it may require depressurization for effective firefighting. Thus, a quick descent below 10,000 feet is essential.
Safe Altitude
Safe altitude information can come from various sources, including:
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ATS Route charts showing MEA and MOCA
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IFR charts with MORA (Minimum Off Route Altitude)
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Topographical maps for refinement
Emergency safe and Minimum Sector Altitude (MSA) data from approach charts also provide valuable information.
Altitude Corrections
Emergency altitudes need corrections for:
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Altimeter temperature errors
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Altimeter pressure settings
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Wind effects, particularly mountain waves
Escape Routes
Escape routes are crafted based on the most restrictive drift down or loss of pressurization scenarios, often defined by the latter. For predefined escape routes, the crew should prepare the following:
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Minimum Route Altitude ensuring safe obstacle clearance
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Route Segment divisions if necessary
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Escape Fix, a predefined starting point for the escape route
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Escape Route detailing the track to follow in emergencies
Vertical Profile
In an engine failure, the crew will turn toward the escape fix and establish a drift down profile. For loss of pressurization, the crew dons oxygen masks and begins an emergency descent. In the event of a fire, the crew accelerates while managing the descent to the minimum route altitude.
Use of Automation
To ensure effective escape route execution, crews must immediately recognize the escape fix, turn direction, and initial safe altitude. Having escape route charts readily available and programming them into the Flight Management System can ease the workload. Utilizing autopilot during emergencies allows the crew to focus on managing the escape profile and coordinating with ATC, especially vital during high-stress situations.
