Cabin Pressure Altitude

[Greg Esres]

14 CFR 121.327 says that "cabin pressure altitude" and "flight
altitude" mean the same thing for unpressurized cabins.

Now, even though Part 91 doesn't have the same statement, I think it
reasonable that the same definition applies to 91.211 Supplemental
Oxygen:

---------------<snip>-------------
At cabin pressure altitudes above 12,500 feet (MSL) up to and
including 14,000 feet (MSL)
---------------<snip>-------------

What this means is that the oxygen requirements kick in based on MSL
altitudes, not pressure altitudes as is commonly thought, at least for
those of us with unpressurized cabins.


================================================== =======
§ 121.327 Supplemental oxygen: Reciprocating engine powered airplanes.
.....
(d) For the purposes of this subpart "cabin pressure altitude" means
the pressure altitude corresponding with the pressure in the cabin of
the airplane, and "flight altitude" means the altitude above sea level
at which the airplane is operated. For airplanes without pressurized
cabins, "cabin pressure altitude" and "flight altitude" mean the same
thing.

[BTIZ]

Run that by me again??

I read it to say "cabin pressure altitudes equivalent to 12500-14000MSL"

I can fly pressurized at FL200 without O2, but if the cabin pressurization
has a problem and "leaks" up to 12,500 (MSL equivalent) or higher, I best
have supplemental O2 handy

And if I am small GA aircraft with no pressurization capability, then the
MSL altitude I'm flying at is the cabin altitude.

BT


"Greg Esres" > wrote in message
...
> 14 CFR 121.327 says that "cabin pressure altitude" and "flight
> altitude" mean the same thing for unpressurized cabins.
>
> Now, even though Part 91 doesn't have the same statement, I think it
> reasonable that the same definition applies to 91.211 Supplemental
> Oxygen:
>
> ---------------<snip>-------------
> At cabin pressure altitudes above 12,500 feet (MSL) up to and
> including 14,000 feet (MSL)
> ---------------<snip>-------------
>
> What this means is that the oxygen requirements kick in based on MSL
> altitudes, not pressure altitudes as is commonly thought, at least for
> those of us with unpressurized cabins.
>
>
> ================================================== =======
> § 121.327 Supplemental oxygen: Reciprocating engine powered airplanes.
> ....
> (d) For the purposes of this subpart "cabin pressure altitude" means
> the pressure altitude corresponding with the pressure in the cabin of
> the airplane, and "flight altitude" means the altitude above sea level
> at which the airplane is operated. For airplanes without pressurized
> cabins, "cabin pressure altitude" and "flight altitude" mean the same
> thing.
>

[Greg Esres]

<<I read it to say "cabin pressure altitudes equivalent to
12500-14000MSL>>

Yes. But this reg has been frequently read to mean 12,500 pressure
altitude. There was a discussion here a while back about why the
"MSL" appeared in the reg; some thought it was a bit of a red herring.

Opinion was divided over whether "pressure altitude" was meant or
"MSL".

For some reason, it now appears quite clear to me the reg means MSL.

[BTIZ]

ah ha... I must have missed that thread...

BT

"Greg Esres" > wrote in message
...
> <<I read it to say "cabin pressure altitudes equivalent to
> 12500-14000MSL>>
>
> Yes. But this reg has been frequently read to mean 12,500 pressure
> altitude. There was a discussion here a while back about why the
> "MSL" appeared in the reg; some thought it was a bit of a red herring.
>
> Opinion was divided over whether "pressure altitude" was meant or
> "MSL".
>
> For some reason, it now appears quite clear to me the reg means MSL.
>
>
>
>
>

[Robert M. Gary]

Greg Esres > wrote in message >...
> <<I read it to say "cabin pressure altitudes equivalent to
> 12500-14000MSL>>
>
> Yes. But this reg has been frequently read to mean 12,500 pressure
> altitude. There was a discussion here a while back about why the
> "MSL" appeared in the reg; some thought it was a bit of a red herring.
>
> Opinion was divided over whether "pressure altitude" was meant or
> "MSL".
>
> For some reason, it now appears quite clear to me the reg means MSL.

I didn't see that thread, the regs have always seemed pretty clear to
be MSL to me. However, your body reacts to pressure altitude (not
density in this case).

-Robert