To be considered airworthy, are commercial aircraft fuselages required to support some amount of external overpressure without buckling?

I was thinking about whether an aircraft is designed to use internal pressure to maintain its rigidity (like some rockets) and then realized that this would probably be a bad idea because then a loss of pressure could be catastrophic. So, aircraft must be able to prove that they can fly when the difference between the inside and outside pressure is as low as zero. But then I started wondering if there might be a good reason why aircraft need to support a negative minimum pressure difference. For example, if the cabin pressure were, for some reason, equalized with ambient at a high altitude, and then the aircraft descended to sea level without re-equalizing pressure, the aircraft would experience a negative pressure difference. Another possibility is that there is a way for a hull breach in a worst-case location to combine with the airflow rushing past the aircraft to suck the air out of the cabin in a way that creates a negative pressure difference.

If either scenario is considered "possible", then perhaps commercial aircraft must be designed and tested to withstand negative pressure differences.

My question is:

a) Are commercial aircraft designed and tested to withstand negative pressure differences, and

b) If yes, how much negative pressure difference are they required to support?

Note: This question, which discusses pressure relief valves, provides a clue but does not clarify what negative pressure differential an aircraft cabin must actually be able to withstand.

• One of the answers certainly answers part (a) provides a clue to part (b). Since the negative pressure relief valve opens at -1 psi, I assume that aircraft must be tested to at least -1 psi negative differential. But it's not a direct answer to part (b) of the question. Commented Jul 10 at 7:42

1 Answer

The certification specifications for large aeroplanes (CS-25) require that pressurized compartments can withstand differential pressure in either direction up to the relief valve setting:

For aeroplanes with one or more pressurised compartments the following apply:

(a) The aeroplane structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting.

(b) The external pressure distribution in flight, and stress concentrations and fatigue effects must be accounted for.

(c) If landings may be made with the compartment pressurised, landing loads must be combined with pressure differential loads from zero up to the maximum allowed during landing.

(d) The aeroplane structure must be strong enough to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1.33, omitting other loads. [...]

(EASA CS-25 - CS 25.365 Pressurised compartment loads, emphasis mine)

The relief valve setting can be different for each aircraft. Typically, the maximum positive pressure difference is much higher (since this is the normal case for pressurized aircraft) than the maximum negative pressure difference.

For example, on the Airbus A320 the limits are +8.6 psi and -1.0 psi respectively:

Two independent pneumatic safety valves prevent cabin pressure from going to high (8.6 psi above ambient) or too low (1 psi below ambient).
They are located on the rear pressure bulkhead, above the flotation line.

(Airbus A320 FCOM - Air Conditioning and Pressurization - Pressurization - Main Components - Safety Valves)

Compliance with the certification specification is not necessarily tested on the full aircraft, if other means are available:

1. Certification Approaches

The following certification approaches may be selected:

(a) Analysis, supported by new strength testing of the structure to limit and ultimate load. This is typically the case for New Structure. [...]

(b) Analysis validated by previous test evidence and supported with additional limited testing. This is typically the case for Similar New Structure. [...]

(c) Analysis, supported by previous test evidence. This is typically the case for Derivative/ Similar Structure. [...]

(d) Test only.
Sometimes no reliable analytical method exists, and testing must be used to show compliance with the strength and deformation requirements. In other cases it may be elected to show compliance solely by tests even if there are acceptable analytical methods. In either case, testing by itself can be used to show compliance with the strength and deformation requirements of CS-25 Subpart C. In such cases, the test load conditions should be selected to assure all critical design loads are encompassed. [...]

(EASA CS-25 - CS 25.307 Proof of structure - AMC 25.307)

• Wow - to test, do you vacuum/pressurize the interior, or, does there exist a sealed tunnel-like thing you can put the whole jumbo in and change the exterior pressure?!?!! Commented Jul 10 at 16:10
• @Fattie I don't think they have a large enough vacuum chamber for that. You can do some tests on the ground with the outflow valve closed, but I guess some tests will have to be done in flight using manual pressurization control. Commented Jul 10 at 17:01
• I see .. actually would it be necessary to do that in flight? Wouldn't it be the same just sitting on the ground? Commented Jul 10 at 17:04
• @Fattie the aircraft couldn't create a negative pressure difference (i.e. lower pressure inside the fuselage) by itself on the ground. It can in the air by maintaining cabin pressure while descending. Maybe they have some device to de-pressurize an aircraft on the ground, but I've never heard of it. Commented Jul 10 at 17:08
• @Bianfable Not always true. The A320, for example, can create negative pressure due to avionics cooling extraction fans. Commented Jul 10 at 18:30