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fatalities or permanent physiological damage to those exposed to such pressure changes. That is,
as the value of the integral increases, the likelihood of fatalities or permanent physiological
damage also increases. The FAA has issued a final version of our Interim Policy which uses a
table of altitudes and cumulative exposure times in lieu of the pressure-time integral. The values
of altitude and time in the table and the results of the pressure-time integral method are in
agreement.
Accordingly, our Interim Policy focuses on minimizing the likelihood that—if a person is
exposed to high altitude cabin pressure from any failure not shown to be extremely improbable—
he will suffer permanent physiological damage. To analyze petitions for exemption from
§ 25.841(a)(2), the FAA requires information about emergency descent rates, any design features
that increase such rates, other design features that offset the inherent increased risk of exposure
to high altitude cabin pressure, and operational procedures.
As stated above and in our Interim Policy, the FAA acknowledges that there is a lack of relevant
data on the effects of exposure to high altitude cabin pressure following decompression and,
particularly, those effects on people of various ages, people with circulatory or respiratory
diseases or certain other medical conditions.
The FAA supports a research program to gather additional information on the effects of exposure
to high altitude cabin pressure. We envision that such research would be conducted in
conjunction with rulemaking to develop a new standard for cabin pressure altitude following
decompression.
8
Our review of the Airbus petition indicates that Airbus used the methodology recommended in
the FAA’s Interim Policy. The FAA believes that this methodology is conservative in the sense
that it assumes a lower partial pressure of oxygen than would likely be present during
decompression at 43,000 feet.
Airbus provided descent profiles for the A380, based on conservative estimates of descent
performance for several failure scenarios, as described in the FAA’s Interim Policy. The descent
profiles indicate that the A380 can descend rapidly from 43,000 feet altitude to below 25,000
feet.
Airbus also performed a depressurization analysis, based upon maximum cruise flight conditions,
defined the envelope of vulnerability of passengers following failures that result in a
decompression, and identified design and operational features of the A380 which would mitigate
the effects of an increase in cabin pressure altitude.
The decompression analysis used several measures recommended in the Final Report of the
MSHWG. Specifically, Airbus estimated the severity of exposure to high altitude cabin pressure
for occupants, based on calculation of a Depressurization Exposure Integral (DEI). The analysis
also considers the relationship between cabin pressure and the Depressurization Severity
Indicator (DSI), a measure of the partial pressure of oxygen. The analysis indicates that the
physiological effect of a slight increase in the length of time spent above 25,000 feet is within the
uncertainty band of available physiological data. The Airbus analysis also shows that—for all of
the failures modes reviewed for this exemption—resultant DSI levels were much less than the
critical value recommended by the MSHWG.
The FAA reviewed information provided by Airbus about design features and operational
procedures that would increase the descent capability of the A380 and/or occupant survival. We
concluded that the design features and operational procedures associated with rapid
decompression followed by an emergency descent support grant of an exemption.
3. Review of historical data and research
FAA reviewed databases from our own National Aviation Safety Data Analysis Center, covering
1959 to the present. Since 1959 there have been approximately 3,000 instances of
loss-of-cabin-pressure. The vast majority of these have been caused by system failures, (e.g.,
cabin pressurization controller failures and valve failures) and structural failures, (e.g., door seal
failures) which have typically been recognized at low altitude within a few minutes after takeoff.
Pilot error has also contributed to the number of events. The majority of these events have not
subjected the occupants to exposures above 25,000 feet (an altitude considered physiologically
significant). Indeed, the cabin pressure altitude in most events did not exceed 15,000 feet (the
cabin pressure altitude at which passenger oxygen masks are deployed).
Similarly, uncontained engine rotor burst failures tend to be very rare. A simple calculation
shows that grouping all engines and transport airplanes together yields an average probability of
　
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