Why the Rules are there - even if it does seem like overkill:
Back in 1968 I was a young Co-Pilot on 84 Sqn (Andovers) at RAF Sharjah. Those were the days when Dhows were built on the beach at Dubai and the only Hotel in Town had 12 Bedrooms and was built of wood.
It was about this time of year when we noticed cliques of Navigators, both from Sqns based at Sharjah and visitors, in corners in deep discussion which abruptly ceased when any member of the "Two Winged Master Race" ventured anywhere near. It was not until the following year that we found out the reason for this. It was the story of the RAF VC 10 which, heading west across the Atlantic, missed America; all of it - South, Central and North America.
The crew were tasked to fly from an airfield in the Midlands to Gander the VC 10 had positioned at that airfield the night before the Task. The Navigator did not sleep well that night partially due to a "Rock Band" performing in another Mess near to the Officers Mess.
At Flight Planning the Navigator found that the North Atlantic Planned Tracks were not available but the Minimum Time Track was so he decided to use that and also to use the Gyro/Grid Technique of Navigation although it was not Command Policy to use that Technique except when 2 Navigators were operating together, with one of them holding an A or B category.
Having routed via Airways to Tory Island and been cleared to fly at FL310 the Nav, having extracted the value of Earth Rate from the tables and added the Residual Transport Wander, set the product -11.8 Deg/hr on the Rate Corrector and switched to GYRO.
Unfortunately, the correct figure should have been +11.8 Deg/hr and the error had the effect of precessing the gyro at a rate of nearly 24 Deg/hr causing the aircraft to diverge slowly north of its intended track.
The first astro fix gave a position close to planned track. It was during the second astro fix that the Nav's attention was drawn to a large return on the CCWRwhich the crew concluded was a large amount of ice. They were correct - it was a large amount of ice, but it was ice attached to the Eastern coast of Greenland because the VC10 was now some 510 NM north of track still locked in a gentle turn to the right.
The second astro fix appeared to be hopelessly wrong so the Nav tuned in the Prince Christian NDB which confirmed his worst fears and an emergency was declared. Aftersome excellent work by USAF Air Defence radar operators the aircraft eventually land safely at Gander with considerably less fuel than intended.
What, however, would have happened if the CCWR had "gone on the blink" a couple of hours outbound? It would have been a "No-Go" item before departure but safely airborne and nearly half way across the Ocean with a benign Weather Forecast, would the crew have returned to the departure airfield or continued?