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Why altitude caps the vacuum
A vacuum bag works because the atmosphere outside pushes on it harder than the thin air left inside. The gauge on the bag reads that difference, and the difference is the pressure consolidating the laminate. With a perfect pump the bag holds nothing, and the gauge reads the full local air pressure. It cannot read more.
Air pressure falls with height. In the standard atmosphere used by ICAO and the U.S. Standard Atmosphere 1976, P = 29.92126 × (1 − 6.87559×10⁻⁶ × h)^5.25588 inHg, with h in feet, up to 36,089 ft. That is about 1 inHg less for every 1,000 ft near sea level.
The FAA Airframe Handbook (FAA-H-8083-31, Chapter 7) notes on its example repair cure cycles: keep a minimum vacuum of 22 inches of mercury during the cure. On a standard day, 22 inHg is the whole atmosphere at about 8,300 ft. A shop at 6,000 ft has 23.98 inHg to work with: under 2 inHg of margin for every leak in the bag.
Worked example
A repair station at 5,000 ft, curing a repair that calls for 22 inHg.
- P = 29.92126 × (1 − 6.87559×10⁻⁶ × 5,000)^5.25588 = 29.92126 × 0.96562^5.25588 = 24.90 inHg.
- Margin = 24.90 − 22 = 2.90 inHg with a perfect pump.
- Ceiling for 22 inHg: h = (1 − (22 / 29.92126)^(1/5.25588)) / 6.87559×10⁻⁶ = 8,266 ft.
Possible at 5,000 ft, but a bag that leaks 3 inHg will not hold 22. Do the leak check before the cure, not during it.
Limits — when NOT to use this
- The standard atmosphere is an average. On a low-pressure day the real figure is lower. A barometer reading of absolute station pressure at the shop is better than the elevation.
- Not the altimeter setting. QNH is corrected to sea level; it is not the pressure in your hangar. Use the absolute (station) pressure.
- Best case only. Real pumps, fittings and bags lose some vacuum. The figure here is the ceiling, not what you will get.
- The requirement comes from the repair data. The 22 inHg default is the Handbook's example. If your SRM or material data sheet gives another figure, use it, and if the shop cannot reach it, the answer is an engineering question, not a lower number.
Questions people actually ask
My gauge reads 25 inHg at 5,000 ft. Is that possible?
Not on a standard day: 24.90 inHg is all the air there is. A reading above the local atmosphere points to a gauge out of calibration, or to an unusually high-pressure day. Check the gauge.
Does a bigger pump help at altitude?
It reaches the ceiling faster and copes better with small leaks. It cannot raise the ceiling: the ceiling is the atmosphere.
What about kPa or bar?
Switch to metric at the top. 1 inHg is 3.386 kPa, so 22 inHg is 74.5 kPa of vacuum and sea level is 101.3 kPa.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 7 (Advanced Composite Materials) — the example cure cycles: “Keep a minimum vacuum of 22 inches of mercury during the cure cycle”.
- U.S. Standard Atmosphere, 1976 (NOAA, NASA, USAF) — sea-level pressure 29.92126 inHg (1013.25 hPa) and the troposphere lapse rate of 6.5 °C/km from which the pressure–height formula follows. Identical to the ICAO Standard Atmosphere (Doc 7488) below 32 km.