Font Height vs. Text Height on aerospace placards

25th July 2020

This is a pretty niche but problematic issue. Engineering drawings for aerospace parts clearly need to be accurate. Dimensions must be absolute; no room for error; nothing left to interpretation… until you get to size of text on labels and placards. Ultimately, if it’s wrong, the plane will still fly, but safety information that is too small can be dangerous. It also means your quality inspectors may reject a placard that is correct, or approve a placard that’s incorrect. It’s all because text height and font height are different measurements.

Below is Helvetica bold, a very common type face used within the aerospace industry. This first example is set at FONT HEIGHT of 10mm. However, the capital ‘L’ actually measures 7.2mm high. This is because a font height is usually set by the top of the highest letter (commonly a capital letter or lower case ‘L’ and the bottom of a lowercase ‘G’ or ‘J’). I say “usually” because some fonts heights also include extra spacing above and below the letters to allow for text above and below it. There is no rule for how to set font height when fonts are designed.

Below is Helvetica bold with a TEXT HEIGHT at 10mm. This is the measurement of a uppercase character. In uppercase, this all works fine, but in lowercase the P’s fall below the 10mm line.

Non-Latin characters, such as Chinese text have different rules again.

To ensure the text size is fool-proof for both engineer, placard manufacturer and quality inspector, the best way to draw them is as follows.

  1. State the chosen font on the drawing
  2. Take a measurement from the top to the bottom of a capital letter and mark this dimension on the drawing.
  3. Where non-Latin text is used (such as Chinese characters), measure the uppermost and lower most point of the first character and mark that dimension on the drawing.

If the correct font is the used, the text can be sized by the placard manufacturer exactly as you want it and your quality inspector will have a clear measurement to check against. No delays, no NCRs, just shiny placards and labels to apply to your parts.

About Teklagraf

We’re a leading manufacturer of aluminium, polycarbonate, vinyl, brass and steel labels and placards to the aerospace, defence and marine industries. Teklagraf pride ourselves on getting things right first time. To find out how we take the pressure from our buyers and achieve incredibly fast turn-around times, get in touch and see how we’re taking labels and placards to new heights.