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If I understand correctly, the mechanism to inflate an emergency slide relies on a tank of inert gas and on an "aspirator" that sucks ambient air. This aspirator works thanks to depression causes by the stored inert gas velocity (Venturi effect) while it inflate the slide.

This implies that the slide requires enough outside air to inflate correctly. I'm confident such mechanism are certified to work at high altitude international airport (I'm thinking of Lima but there may be other international airport at high altitude), but an aircraft can still crash in high altitude mountains, higher than any airport.

I wonder what is the minimal outside pressure required to inflate an emergency slide. Maybe air density is more relevant than pressure. Given the standard atmosphere model, both can be converted in altitude (either using pressure-altitude or density-altitude).

What is the maximal altitude at which the mechanism to inflate an emergency slide can work as expected (i.e. suck enough outside air to inflate the slide)?

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    $\begingroup$ Maybe you should consier replacing "altitude" with "elevation" in the title, since altitude implies the aircraft is flying $\endgroup$ – expeditedescent Aug 21 '20 at 14:03
  • $\begingroup$ @expeditedescent I think altitude is correct in this instance, as altitude is in reference to the static air pressure (which changes), whereas elevation is in reference to the ground height above mean sea level. The question is in relation to the surrounding air pressure. $\endgroup$ – selectstriker2 Aug 21 '20 at 17:37
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    $\begingroup$ While it may be technically correct, I still think it is confusing. $\endgroup$ – expeditedescent Aug 21 '20 at 18:12
  • $\begingroup$ @expeditedescent I don't know which term is better. I'm really interested in air quantity. If density is important, then I'm interested in density-altitude; if pressure is important, then I'm interested in pressure-altitude. As each of those terms use "altitude", I though "altitude" is better. But as English in not my mother tong, I surely fail to understand which one is better and why. $\endgroup$ – Manu H Aug 21 '20 at 18:39
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In the US, TSO-C69c details the FAA's technical requirements and testing protocols for evacuation slides. I can't find anything in it that talks about a maximum altitude or minimum air pressure. The closest thing I can see is this requirement in 4.2 to function correctly in a given temperature range:

The device, including its inflation system, must be demonstrated to be capable of fully functioning when subjected to temperatures from -40 to +160 degrees F. If the device is intended for installation outside the pressurized cabin, the device must be capable of functioning after being stowed at -65 degrees F. The function of the device must be demonstrated in accordance with the hot and cold soak test procedures described in paragraph 5.9 of this appendix.

Sections 4.11 on inflation operation and 4.15 on the inflation system don't mention anything about ambient atmospheric pressure.

And the testing protocols in section 5 of the document only mention temperature, not pressure. For example, section 5.9.3 is part of the cold soak test protocol:

Deploy the device into ambient temperature conditions (typically defined as between 65 and 85 degrees F) from the appropriate airplane door or a suitable airplane door mock-up or module, within 10 minutes after removal from the conditioning chamber.

4.15.2 has the only reference in the document to an aspirator system and it only requires a filter to avoid ingesting small objects.

So, unless I've missed something in the TSO - which I might have! - maximum altitude/elevation or minimum ambient atmospheric pressure aren't part of the required certification, at least in the US.

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The design (as described by the OP) is inherently self-compensating, as all good systems should be when circumstances permit. At higher altitude, which implies lower DA (let's just say density) of the outside air, the venturi-aspirator device will pull a lesser mass of air into the slide as it operates. But also,the outside air pressure is lower than at sea level hence the slide requires less of the air-gas mixture to fill completely and reach an adequate level of over-pressure to remain firmly inflated. That aside, I am sure that back in the days when regulators were doing their jobs properly and checking engineering work, they would have picked up altitude variations as an issue while conducting HAZOPs and worked through the issues. That will be why altitude does not show up as an operational limitation.

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  • $\begingroup$ You know, the design is the one described by wikipedia (I may misunderstand how it works). If I understand correctly, you're basically saying that the quantity of air suck by such device decreases at the same speed as (or slower than) outside pressure and thus compensate. $\endgroup$ – Manu H Aug 28 '20 at 7:07
  • $\begingroup$ Yes Manu H, that is the intended meaning of my response. By the way, I support the use of the word altitude in your original question, as opposed to elevation. What we are specifically interested in here is Pressure Altitude, insofar as it influences performance of the inflation system. Elevation is irrelevant - it merely references the "parking place" of the unfortunate aircraft that is undergoing emergency evacuation. When discussing aviation we should use correct aviation terminology - as you have. $\endgroup$ – JSB-PP Oct 11 '20 at 21:06

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