The excerpt gives a clear sequence — Page 144, Lesson 224

I want to walk you through the procedures and error categories that apply when you're operating inside the North Atlantic Minimum Navigation Performance Specification Airspace — the MNPSA. This is the high‑traffic, high‑precision airspace over the North Atlantic where lateral and longitudinal separation standards are reduced, so any navigation deviation has serious consequences. Let's start with the actions a pilot must take if a navigation system failure occurs after entering the MNPSA. The excerpt gives a clear sequence. First, you assess the situation. Then you prepare a proposal to put to the OACC — that's the Oceanic Area Control Centre, the air traffic control unit responsible for that oceanic sector. You advise and consult with the OACC, and you obtain an appropriate re‑clearance before you deviate from your previous clearance. That's critical: you cannot just turn or change altitude on your own; you need a new clearance first. While you continue the flight, you should closely monitor the remaining navigation system that's still working. You cross‑check your heading information with the magnetic compass — the mag compass — and with visual indications such as other aircraft tracks and contrail direction. You should also try to establish communications with another aircraft that you can see, and cross‑check heading information with that other aircraft. This is all about using every available source to confirm your position and heading when your primary navigation is degraded. Now, if the remaining system also fails after you've already entered the MNPSA, the pilot's actions escalate. You notify the OACC immediately. You make best use of the information you've already obtained from other aircraft. You keep a good look out for other traffic. You make maximum use of exterior lights to increase your visibility. You consider climbing or descending 500 feet — that's a standard vertical offset to reduce collision risk when you lose the precise track‑keeping capability. And if the navigation computer itself has failed, you revert to manual navigation. Let's move to the errors associated with oceanic clearances. The excerpt tells us that navigation errors in this environment fall into several categories, and the two most significant are ATC system loop errors and waypoint insertion errors. An ATC system loop error is any error caused by a misunderstanding between the pilot and the controller regarding the assigned flight level, Mach number, or route to be followed. So it's a communication breakdown. These errors can arise from incorrect interpretation of the NAT track message by dispatchers — NAT stands for North Atlantic Tracks, the organised route structure. They can also come from errors in co‑ordination between OACs — that's Oceanic Area Control centres — or from misinterpretation by pilots of oceanic clearances. Errors of this nature are normally detected by ATC from pilot position reports and will be corrected. But the excerpt warns that timely ATC intervention cannot always be guaranteed, especially because it may depend on the use of third‑party HF radio — high frequency — or even voice communications, which can be unreliable over the ocean. The second major category is waypoint insertion errors. Experience has shown that many of the track‑keeping errors in the NAT MNPS Airspace occur because crews program the navigation system with incorrect waypoint data. These are called waypoint insertion errors. They frequently originate from — and the excerpt cuts off there, but the point is clear: a simple data‑entry mistake when typing a waypoint into the FMS or GPS can put the aircraft miles off the cleared track. So to summarise: in the MNPSA, if a system fails, you assess, propose, consult, and get a re‑clearance before deviating. You cross‑check with the mag compass, visual cues, and other aircraft. If the last system fails, you notify OACC, use external lights, consider a 500‑foot altitude change, and go to manual navigation. And the two big error types to watch for are ATC loop errors — misunderstandings about the clearance — and waypoint insertion errors — typing the wrong coordinates into the box.