First, let's be clear on the situation — Page 141, Lesson 222

I want to walk you through the procedures for when you're operating in the MNPSA — the Minimum Navigation Performance Specification Airspace — and you run into a situation where you cannot get a revised ATC clearance. First, let's be clear on the situation. You're already flying in the MNPSA, and for whatever reason — maybe radio congestion or a frequency issue — you cannot immediately obtain a revised clearance from air traffic control. The rule is straightforward: if you can't get it right away, you must obtain it at the earliest possible time. But in the meantime, while you're waiting, there are four actions you as the pilot must take. First, broadcast your position and your intentions on 121.5 MHz at suitable intervals. That's the international VHF emergency frequency, and you keep repeating that broadcast until you finally get that revised clearance. Second, make maximum use of the aircraft lights to make your aircraft visible to other traffic. Third, maintain a watch for conflicting traffic — you are actively looking out the window. Fourth, initiate such action as necessary to ensure the safety of the aeroplane. That's a catch-all: you do whatever is needed to stay safe. Now, if you are unable to obtain a revised ATC clearance at all, the aeroplane should leave its assigned route or track by turning at least 45° to the right or left, whenever this is possible. The direction of that turn should, where possible, be determined by the position of the aircraft relative to any organized route or track system. Other factors you consider include the direction to an alternate aerodrome, terrain clearance, and the levels allocated to adjacent routes or tracks. What you do next depends on your ability to maintain your assigned flight level — that is, whether you can hold the altitude you were cleared to fly at. If you are able to maintain your assigned flight level, here is the sequence. First, turn to acquire and maintain in either direction a track that is laterally separated by 15 nautical miles from your assigned route or track. That means you move 15 NM sideways off your original path. Then, depending on your altitude: if you are above FL410, climb or descend 300 metres, which is 1000 feet. If you are below FL410, climb or descend 150 metres, which is 500 feet. If you are exactly at FL410, you climb 300 metres — 1000 feet — or descend 150 metres — 500 feet. So the action is altitude change plus lateral offset. If you are unable to maintain your assigned flight level — meaning you cannot hold that altitude, perhaps because of an engine problem or pressurization failure — the procedure changes. First, initially minimize your descent rate to the extent that it is operationally feasible. You don't want to drop like a stone; you control the descent as gently as possible. Then, while descending, turn to acquire and maintain in either direction a track laterally separated by 15 NM from your assigned route or track. So you offset sideways while you're going down. Then, for the subsequent level flight — once you stabilize at a new level — you select a level that differs from those normally used by 300 metres — 1000 feet — if above FL410, or by 150 metres — 500 feet — if below FL410. So you pick an altitude that is not the standard one for that direction of flight, to reduce the chance of conflict. Now let's move to the next major topic: navigation system failure in the MNPSA. If a failure occurs before you enter the MNPSA, the pilot should attempt to resolve the problem — that is, determine that the system is still useable — or land, or delay the take-off, and get the system repaired. Under no circumstances should an aircraft enter the MNPSA with unresolved navigation errors. That is a hard rule: you do not go in if your navigation is not working properly. Alternatively, you may consider obtaining a clearance to remain outside the MNPSA — for example, below FL290 — or re-file for one of the special routes designed for non-compliant aircraft. Those are routes that do not require the full MNPSA navigation performance. However, if your aircraft has triple LRNSs — that's three Long-Range Navigation Systems — then the MNPSA requirement is for two serviceable LRNSs. So with three on board and one unserviceable, you still meet the requirement with the remaining two, and the flight can proceed as planned. If an aircraft with only two LRNSs suffers a system failure after entering the MNPSA, the flight is to be continued in accordance with the clearance received. In this case, the pilot should — and the excerpt cuts off there, but the key point is you continue on the clearance you already have, and you do not have the redundancy to lose another system.