 |
Figure 13-1. Simulated Cabin Altitude versus Time.
Go
to: Interim
Policy, MSHWG Report, pAO2
Analysis, Analysis Programme
Analysis
of cabin pressure versus time is required to show compliance to the latest
revision to FAR 25.841.
When
FAR 25.841(a) was revised as part of Amendment 25-87 in 1996, it became apparent
that application of this rule would have enormous impact on the future design of
commercial transport aircraft. Analyses
showed that, for a wing-mounted engine aircraft, compliance to the new rule
would limit the maximum altitude to 35,000 to 37,000 ft.
Obviously, this is considerably lower than existing wing-mounted engine
aircraft (currently, 43,000 to 45,000 ft), and in an altitude regime that does
not utilize the efficient aerodynamics of the aircraft.
Future aircraft could fly sub-sonically to altitudes as high as 51,000
ft, which is where many of the executive jets are certified to fly.
An
Aviation Rulemaking Advisory Committee (ARAC) assigned a Mechanical Systems
Harmonization Working Group (MSHWG) to perform the task of re-writing the rule
for any sub-sonic aircraft capable of flying at altitudes up to 51,000 ft.
I was a non-voting member of this MSHWG from May 2001 until retirement in
July 2003.
In
March 2003 the FAA proposed a Time-Integral format (with my help) in an Interim
Policy proposal using known and projected medical data pertaining to lung Oxygen
partial pressure (alveolar pressure – pAO2). The pAO2 data up to 25,000 ft altitude was obtained from Dr.
Roy L DeHart’s book (“Fundamentals of Aerospace Medicine- second edition
1996) using Table 5.12 on page 91. Alveolar
partial pressure was extrapolated arbitrarily by the FAA to be zero at 87-mmHg
absolute pressure (47 mmHg water vapour plus 40 mmHg CO2 in the lungs).
(This means zero partial pressure for all gases including oxygen).
The
Time-Integral was based on experimental data performed by Nicholson, Ernsting
and Brierley in 1967 and 1969 on baboons and monkeys. Analysing the integral value data for values of pAO2 < 30
mmHg, it was discovered that there was a cutoff value of the Integral
corresponding to survival versus non-survival of the non-human primates.
From this data a measure of the severity of exposure to aircraft
occupants was established.
The
FAA simplified the Interim Policy by providing Descent Rates versus Initial
Operating Altitude in a Table.
Descent Rate versus Operating Altitude Table |
|
|
Average
Emergency Descent Rate - fpm |
Maximum
Operating Altitude - ft |
|
6,000 |
40,000 |
|
7,000 |
42,500 |
|
8,000 |
45,000 |
You
can see the complete FAA Interim Policy at TBD
The
following is a direct quote from the MSHWG report:
A
means of compliance to the requirements of 25.841(a) may be demonstrated through
the use of the depressurization exposure integral method described herein that
provides the quantitative means to ensure that an airplane design meets the
intent of the regulation with respect to protecting human physiology following a
rapid decompression. The criterion relies upon the use of the Depressurization
Exposure Integral (DEI) method. The foundation of the DEI method is that while human
physiological response to a rarefied environment is a dynamic multi-variable
problem, the two parameters of dominance are the pressure that the subject is
exposed to and the duration of that exposure.
Qualitative means could be utilized to assess risk to occupants but the
uncertainty of the level of risk necessitates that specific features be
incorporated into airplane designs to enhance survivability and lower the risk
to the occupants.
The
theoretical basis of this approach rests with the results of animal
decompression studies, “Neurological Sequelae of Prolonged Decompression”,
Aerospace Medicine, A.N. Nicholson and J.R. Ernsting, April 1967, and
“Neurological Study of Simulated Decompression in Supersonic Transport
Aircraft”, Aerospace Medicine, J.B. Brierley and A. N. Nicholson, August 1969
(References 4 and 5). Figure 13-1
shows the chamber pressure (in mmHg) time history from Reference 5 for both of
these experiments [pressure altitude in feet versus time in minutes]. This data provided critical information needed to establish a
measure of severity to the occupants of an airplane in the event of a sudden
loss of pressure. The first step
was obtaining a relationship called the Depressurization Severity Index (DSI).
This relationship provides a measure of the severity of the
depressurization to atmospheric total pressure and was determined from published
data [Reference 6] and calculation, see Figure 13-2.
Figure 13-1.
Simulated Cabin Altitude versus Time.
Figure 13-2.
Depressurization Severity Index (DSI) as a Function of Cabin Pressure
Figure
13-2 serves as a mathematical transfer function that converts input from Figure
13-1 [Simulated Cabin Altitude versus Time] into Figure 13-3 [Depressurization
Severity Index (DSI) versus time]. The equations that describe the values of DSI
as a function of cabin pressure are as follows:
For Pcabin >= 411.89 mmHg then DSI
= (0.1752 * Pcabin) - 30.176 [mmHg]
(based on Table 5-12, pg. 91, Reference DeHart, 2nd Edition.)
For Pcabin >= 291.995 AND Pcabin < 411.89 mmHg then DSI = (0.0004441 * Pcabin ^2) - (0.2209 * Pcabin) + 57.625 [mmHg] (based on Table 5-12, pg. 91, Reference DeHart, 2nd Edition.)
For Pcabin >= 47 AND Pcabin < 291.995 mmHg then
DSI = (16.964 * LN(Pcabin)) -
65.312 [mmHg] (logarithmic extrapolation to zero at 47 mm Hg (62,810 feet altitude))
For Pcabin < 47 mmHg then DSI = 0 [mmHg].
In
addition, Figure 13-3 includes data from the experiment by Dr. Hans Clamann,
(Reference 8), which provides additional corroboration of this approach.
Dr. Clamann utilized a chamber to simulate a rapid decompression from
9,800 feet to 49,200 feet (pressure altitude) and then repressurized the chamber
at a rate of 24,600 feet per minute (simulating an airplane rate-of-descent). He
did not use supplemental oxygen but breathed air at the chamber pressure.
It was reported that he retained consciousness during the entire, albeit
short, event.
Figure 13-3.
Hypobaric Chamber Experiments, Time Variation of DSI
Using
the relationship in Figure 13-2, the calculated DSI time history for the
experimental results given in Figure 13-1, are presented in Figure 13-3.
Historically FAA has referenced 10,000 feet [approximately equivalent to
DSI of 60 mmHg] and 25,000 feet [approximately equivalent to DSI of 30 mmHg] as
critical points in the cabin pressure altitude, and these were selected as
reference conditions. Integrals of
the time history of the DSI, defined as Depressurization Exposure Integral
(DEI), below 30 mmHg and 60 mmHg provide a measure of the severity of the
depressurization event.
DEI
value below 30 mm Hg is defined as DEI30, DEI value below 60 mm Hg is defined as
DEI60.
It
is observed that a direct correlation of the DEI to increasing likelihood of
fatalities or permanent physiological harm being sustained by the subjects
exists [Figures 13-4 and 13-5]. For
example, the experimental data resulted in values ranging from 2,779
mmHg-seconds to 22,241 mmHg-seconds for the integral below 60-mmHg.
In addition, data from the experiment by Dr. Hans Clamann as reported in
“An Analysis of the Oxygen Protection Problem at Flight Altitudes Between
40,000 and 50,000 Feet, Final Report”, prepared for the Federal Aviation
Agency, Contract FA-955, by Blockley and Hanifan, February 20, 1961 (Reference
8) provides additional corroboration of this approach.
Dr. Clamann utilized a chamber to simulate a rapid decompression from
9,800 feet to 49,200 feet (pressure altitude) and then repressurized the chamber
at a rate of 24,600 feet per minute (simulating an airplane rate-of-descent). He
did not use supplemental oxygen but breathed air at the chamber pressure.
It was reported that he retained consciousness during the entire, albeit
short, event, Reference 8.
End
of quote.
You
can see the complete MSHWG Report at TBD
The
equations to use are, again (quoted in revised AC25-20):
Pcabin >= 411.89 mmHg then:
Pcabin >= 291.995 AND Pcabin < 411.89 mmHg
then:
Pcabin >= 47 AND Pcabin < 291.995 mmHg then:
Pcabin < 47 mmHg then:
AC25-20
also gives values of Discharge Coefficient (Cd) as being:
* 0.50
for all holes (except windows)
* 0.75
for windows (however, if of triplex design can be assumed to be extremely
improbable 1E-9)
A
programme can be written to perform the pAO2 analysis, and in addition to the
above, should take into account the following pertinent aircraft parameters
(constant values for any given aircraft):
* Aircraft volume
* Aircraft Air Conditioning Pack flows (with appropriate
failure case for uncontained engine failure)
* Additional aircraft leakage (including closed Outflow Valve)
I have already written this programme (developed during the MSHWG process to verify our conclusions), and am willing to perform analyses for any aircraft manufacturer based on receiving the appropriate parameters for the aircraft, and at the quoted consultation fee.