Cabin Pressurisation Question!!!!

[Midland Maniac]

Can anyone help me out???

I have passed my systems exam, but there is something about a\c pressurisation that I don't understand. Could anyone help me in answering these questions???

1) How does an airliner landing at airfields such as La Paz in Bolivia (ELEV 13500ft) cope with pressurisation and de-pressurisation??

2) How do they prevent the 'rubber jungle' from deploying on when de-pressurising as the system is set to activate at 13,000ft cabin alt!!!!!

If someone could answer these questions it would be most helpful...maybe then I can start to sleep at night....I obviously have too much time on my hands!!!!

[GlueBall]

The Pressure Controller (either Auto or Manual mode) is set to Field Elevation and local Altimeter (QNH) setting at beginning of descend. When the airport elevation is higher than cabin altitude in cruise (about 5000 feet @ FL350), then going into La Paz would require the cabin altitude to be "raised" during the descend. Passenger oxygen mask auto deployment circuit breaker will have to be pulled to preclude mask deployment; known in some jets as "oxygen avoidance" system, a switch rather than a circuit breaker. :cool:

[atomic]

Not all airplanes can just fly into airports like that.
When I take a look and check my flight manual for the A320, it says that my company (not Airbus)limits the airplane operation to a maximum of 8500' pressure altitude for take off and landing.
I guess the reasons for this are many: Cabin altitude and o2 mask deployment, rules and regulations/ ops specs etc.

[Midland Maniac]

Thanks guys for the info...so are we saying that only certain aircraft are allowed into airports like that???

Do they have to be specially certified??

Does anyone work for an airline that operates into airports such as these??

[QAVION]

"Does anyone work for an airline that operates into airports such as these??"

American Airlines flies 757's into La Paz, I believe. They have a special switch on the overhead control panel.

"Do they have to be specially certified??"

Without the switch (and associated wiring and Cabin px system interface), they obviously couldn't do this. All wiring/system mods require proper certification.

Rgds.
Q.

[CaptainSandL]

The following account is of 737 operation into La Paz an is taken from http://www.b737.org.uk/lapazairport.htm


Here at Varig, we have a daily flight (exc. Sun) from SBGL (Galeão, RJ) to SLLP (La Paz) with stops in SBGR (São Paulo Int'l) and SLVR (Santa Cruz de La Sierra, Bolivia). It's a routine flight until SLVR.

From SLVR some procedures apply:

Flight time to La Paz is 50 minutes. We have to use a B737-300 with 22k engines and 225mph tires. Pilots must be qualified by a Captain, with previous experience for this flight, before a "solo" flight. It's required to make a full IRS alignment and check if some specific positions are in data base. For airplanes with CPCS (pressurization system panel) set SLVR altitude minus 200ft before departure and after takeoff set cabin altitude to 9000ft. Over position TORAX (about 55nm to La Paz) adjust cabin to 12900ft for landings on runway 10 or 13100ft for rwy 28. For digital pressurization control you can use AUTO mode and set LAND ALT to 9000ft. Over TORAX use same altitudes described above. When cabin altitude reaches 10000ft pilots must wear oxygen masks (Norm position) until shutdown checklist.

It's forbidden to make weather deviations during descent, if necessary, maintain cruise level, make any deviation required and descend in a holding pattern over La Paz VOR, obviously following a coordination with ATC. (Unfortunately an Eastern Airlines B727-200 crashed during a descent, making weather deviations. This airplane hit a 21184ft-high mountain in January 1985). At least, just prior to descend, PNF must set VHF NAV to manual and EHSI mode in Exp or Full raw data. FL 230 is the minimum altitude until 20nm to PAZ VOR, after 20nm it's possible to descend to 18000ft and start a VOR/ILS procedure (No radar vectors). Inhibit GPWS (Flap) because only flap 15 is approved for landing due to landing climb limits and do not use autobrake system. Unless any abnormal condition occurs, proper pilot technique is "Maximum reverse thrust, within engine limits, and minimum brakes", this is a 4000m-long runway, and this procedure works well and brakes and tires do not warm and affect turn around (anyway you need to check "Max quick turnaround weight table" after landing). Go around and max takeoff N1 values must be checked using an special table if required.

Runway 10 should be used even with light tailwind (within limits and checking tire speed limit, too) because there isn't an instrument procedure for runway 28 (just circling), and it isn't comfortable to make a circling approach to runway 28 due to a turbulent air near threshold (it's very close to a high terrain, and you fly over a valley where city of La Paz is and downdrafts are very usual). Missed approach for runway 10 is a sharp right turn to avoid Ilimane Peak, described before (Eastern).

For departure, full alignment and oxygen masks are also required and "High altitude airport start procedures" must be followed. Rolling takeoff is not permitted. There are special engine failure climb patterns, for rwys 10 and 28. Improved climb, flaps 1 takeoff and no engine bleed must be thoroughly checked to take the best weight and performance advantage. Performance data is based on AFM Appendix 25 and 37LP.

There are also special enroute procedures and charts to fly in/out La Paz in case of pressurization failure and engine inoperative (with FMC on or off) and some additional details.

The most amazing thing is to make this flight during a clear weather. During initial approach you fly over high terrain with a lot of snow and it's possible to see, not so far, Titicaca lake, but certainly you become very impressed when turning on final to land on runway 10 and the big wall (Ilimane peak) is just behind. An outstanding view !!!

The airplane reaction is very slow and you always must be prepared in advance. Due to the high altitude TAS is higher than usual, but engine response does not correspond as we are accustomed on a sea level flight.

[aluminum ovcst]

I have flown a 737-200 into LPB many times. The account above is very good and comprehensive, and I'd like to add a few points relative to our operation. Our 200s did not have a switch inhibiting deplyment of O2 masks. Indeed, flight crew wear masks above 10K cabin alt, and prior to starting approach, pressurization panel was set to no engine bleeds while APU provides bleed air to keep cabin alt below 14000 ft. This configuration is kept until after landing, in order to allow APU generator to power the busses. Other details I recall are that 1 engine is started at the gate (APU providing bleed and ground cart providing AC), follwed by pushback and crossbleed start. No bleeds takeoff is performed, and immediately after setting climb thrust, packs are reestablished before APU runs out of air and 02 masks pop down. Engine surges are common during spool up with even a slight crosswind and that 4000m runway seems like nothing when taking off with a full load, (about 40 pax on a typical day). Scariest takeoffs I've ever witnessed. Runway 28 (downhill) is preferential for TO, up to a 6 kt tailwin. 7 or more, runway 10, uphill, allows for greater payload). Very interesting operation for an aircraft which was definitely not built with LPB in mind. Aircraft with larger engines pose lesser problems, I imagine. (PS. only -17 engines are certified).

[ 27 September 2001: Message edited by: aluminum ovcst ]

[QAVION]

"Our 200s did not have a switch inhibiting deplyment of O2 masks."

So the cabin altitude trigger point for mask deployment was greater than the baro alt of La Paz?

"APU provides bleed air to keep cabin alt below 14000 ft."

Would this be the trigger point? I ask this because a typical 747-400, for example, has a trigger point between approx 13,250 and 14,500', and La Paz is about 13,300', so would not be able to fly into this airport (even if the runway was long enough). I'm not even sure that the 747-400 APU could provide sufficient bleed air at this altitude to maintain cabin pressure (Especially when you started opening cabin doors for passenger embarkation :D ).

Did you also have automatic emergency decompression PA announcements on the 737-200?

Thanks,
Rgds.
Q.

[Midland Maniac]

Thanks guys, I think that has answered my question!!! Didn't expect such a great response!!!!

Thanks to all those that have replied....

MM :) ;) :p :) ;) :p

[GotTheTshirt]

Qavion,

Isnt the O2 baro switch inhibited with gear on ground ??
If so the apu would just be to keep the 14k until touchdown.

[QAVION]

"Isn't the O2 baro switch inhibited with gear on ground ??"

I think we would have to look at this on a case by case (airplane by airplane) basis, "GTT". For example, the 747-400 O2 automatic deployment system is based on a simple mechanical aneroid (no electrics). The system can be manually activated (via electrics) however if that fails. Having said this though, the movement of the aneroid (capsule), does complete electrical circuits for things like No Smoking signs, automatic PA announcements, etc. Once the system has been activated, the rush of pressurized oxygen into the distribution system unlatches the covers on the overhead mask stowage boxes and the masks fall down (another mechanical system with no option for an air-ground input).

I may be able to find further information on a variety of aircraft in our particular fleet, but none of these, as far as I know, is specifically designed for visiting La Paz. Now that I think about it, however, I would say that aircraft fitted with chemical O2 generators at each seat(which are activated electrically), would be better suited to landing at La Paz (as an air/ground input could easily be inserted into the activation circuit). Perhaps "aluminum ovcst" 's 737-200 had this type of O2 system?

Rgds.
Q.

[ 30 September 2001: Message edited by: QAVION ]