Air Ministry Safety Height Procedures

[Pontius Navigator]

Thought the following on the AM safety height procedures might be vaguely interesting.

In the 60s AMOs was a very thin order book, the heavily thing was there cover. One order covered in great detail, and examinable in various exams was the AMSHP.I

First you had to draw your route on a succession on maps. Next draw 30 mile radius circles around each waypoint and draw team lines of a 60 mile corridor. There was no allowance for a mountain behind you, that would be included. You would then search for the highest contour in this area. You would then add the contour interval on the assumption that the ground rose to the next contour minus one foot. If there was a mast, add that too.

Next add 10%, then add 1,500 feet, and you got your leg safety altitude.

Once airborne the fun started.

If you were in a rapid fixing area (every 6 minutes) that was fine as your position accuracy was assumed at 2 miles per fix.

In a slow fixing area you would apply the fix accuracy which for a single astro position line MPP was about 15 miles and a system accuracy of 20 miles per hour.

Now you had to create a bicycle chain picture - terminal 30 miles, add 20 miles per hour from last fix, add fix accuracy, and you could end up with a safety zone of 60 miles radius or more. All the time you were also trying to get that all important fix to reduce the area of uncertainty.

It was particularly designed for slow piston aircraft in IMC, a not improbable circumstance, or even VMC between layers.

After a long oversea flight your area of uncertainty was indeed large. Modern nav aids such as a GPI and Doppler reduced the accuracy degradation and for maritime aircraft spending hours over the sea we even had a device for adding Surface Movement Compensation.

I don't know of anyone hitting high ground when they knew they were lost, only when they thought they knew where they were.

[Sisemen]

And now one whiles away the time on a boring sector by trying to keep within + or - 1 foot of the GPS track!

[Pontius Navigator]

Slasher mentioned seeing what you want to see.

Some years ago a cruise ship left Bermuda for New York. As the ship sailed her flags were hauled down but in the process the sailor tripped over a cable.

The Sat Nav now being disconnected, the nav display automatically displayed a memory flag. This was not noticed. For the rest of the voyage the ship plot exactly traced the intended course. Well it would wouldn't it.

What check fixes that were taken were near enough to be accepted and not raise any alarm. Eventually, reaching the coast, the radar picture was correlated with the map and all was well until the ship ran aground 16 miles off track.

[Rossian]

Sometimes it wasn't the technology at fault(or lack of it in a Shackleton) human factors came into play.
The two navs had a BIG falling out before we even go airborne about something (40 years have obliterated the root cause) and Nav1 took the top bunk and sulked to sleep. Nav2 left at the nav table fuming made a desultory attempt at looking at the system and announced to the captain that there was a slight problem with the compass which "If I didn't do anything we'd be 100 miles off track at the end of the 12 hour transit back to Ballykelly" and promptly went to sleep, as did most of the crew (It had been a very hard 10 days socialising in PEI). Many hours later the nav2 roused himself when the captain asked for an estimate for Tory Island, he asked radar to look for said island. Nuffin' says radar op. I went to have look myself and asked skipper to climb a bit as there was a hint of land a long way ahead. It became clear we were headed into Galway Bay so a swift change of course away from Ireland and a long trundle north and round the corner to BKY with a very very cross captain. The nav was right we were 100 miles off track!
Harsh words were said in private, as the most junior and newest member of the crew I escaped censure. Phew!

The Ancient Mariner