Typesetting aircraft manuals in the typewriter era

Question

I'm trying to make sense of the table of limitations for the Viper 301 engine. This is from the Dominie T Mk.1 Ground Handling Notes I was fortunate to photograph at Kew a few years back.

The typist has written something like:

Condition         Rev/min per cent
----------------------------------
Ground idling     40 + 10
                     -  0

Does this mean "40% to 50% RPM" essentially? If so, why not "45 ± 5" - the same typist has written ± elsewhere in the same document.

Or does it mean the idling RPM will generally be 40% and might sometimes rise to no more than 50%?

(Yes, I realise I'm quite definitely overthinking this. I'm also wondering how many times this poor typist had to write lb/sq. in. in their career before PSI became an accepted abbreviation...)

edit: ah, this is the reason I doubted my initial assumption - a few pages later the typist has written this:

which I think means 40-41% N1 in modern terms? But they've written 12 - 25 lb/sq. in. just above... so confused.

You're going to be lucky to find anyone who really can answer authoritatively. But doesn't it make sense that if the target is 40%, with 10% greater ok but anything under 40% not ok, that that's a different concept from what you suggested? — quiet flyer, Mar 15 '23 at 23:17
The typewriter/old era isn't important, because it's used in engineering today. A 0 tolerance on one side is called a unilateral tolerance, a range that is different between each side is a unequal bilateral tolerance. — user71659, Mar 16 '23 at 00:12

score 8 · Accepted Answer · answered Mar 16 '23 at 00:37

This is not a typesetting thing, all of these different notations have a different meaning.

$ x\pm y $ means that the nominal ideal value is $x$, and up to $y$ deviation is acceptable.

$ x^{+y}_{-0} $ means that the nominal ideal value is $x$, and up to $y$ deviation is acceptable in the upward direction only. No deviation in the downward direction is acceptable. An example of a modern usage of this is in the instrument PTS:

Maintains the MDA, when reached, within +100 feet, −0 feet to the MAP.

When flying a non-precision approach, the ideal is to fly exactly to the MDA and remain there. Upward deviations of up to 100 feet are allowed, but going below by even a foot is a failure.

A range $x–y$ states that any value between $x$ and $y$ is acceptable, without specifying any specific value as better than others.

Typesetting aircraft manuals in the typewriter era

1 Answers1