An aircraft subjected to unlawful interference (hijacked) and forced to divert without ATS communication shall try to:

• A. Fly the emergency triangle
• B. Declare an emergency
• C. Continue at an altitude that differs from the semicircular rule with 1000 feet when above FL 290 and 500 feet when lower than FL 290
• D. As soon as possible commence emergency descent in order minimise the difference between cabin pressure and outside pressure

Answer: (C)

When a flight is subjected to unlawful interference and there is no communication with air traffic control, the priority is to maintain a controlled, predictable path while awaiting possible contact or resolution. The semicircular rule sets standard altitudes for eastbound and westbound operations to maintain orderly separation. By continuing at an altitude that intentionally differs from these standard levels, the crew preserves separation from normal traffic patterns and avoids being locked into a predictable, easily tracked level if ATC later tries to piece together the flight’s position. The prescribed offsets—1000 feet above the semicircular rule when above FL290, and 500 feet below it when below FL290—provide a safe, deliberate deviation that keeps the aircraft within controlled airspace while not following the standard pairing. This approach is chosen over immediate emergency descent or declaring an emergency because it sustains flight safety and stability under radio silence, giving the crew time to establish contact or reach a safe resolution.