r_stopnicki

High Altitude Airports - Pressurization

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Hello to all!

You just might find this interesting and want to give it a try.

Flying in and/or out of high altitude airports, requires some careful review and operation of the cabin pressurization when the airport's elevation is higher than 8000 feet (I used to be "good" at it in my B767 days, but that was two versions of simulators ago and I cannot recall the details).

Just flew into SLLP (La Paz - El Alto), and its 13300 ASL elevation, on a B747-400F.  Lots of noise from alarms, telling me that this is very high (no kidding)!! These alarms are very common on all aircraft that are landed or initially parked at this airport.

After parking, I was unable to open any of the cargo doors (any door as a matter of fact). One APU gen was on, the other was "available", chocks were there.......but, no door opened. I was afraid of posting here, because I have learned my lesson and I dread Kyle's wrath ;-)

So, I checked the FCOM. Pressurization Systems - Cabin Altitude Control - Page 2.10.21

Of course! The cabin was at 8000 ft and the guys waiting outside with the equipment, were at 13300 feet!

I manually depressurized the cabin and, presto!, everything worked.

So, included in all the checks we normally do before departure or before landing, when the airport is a high altitude one, there is need to verify if the auto system has properly accepted/set that altitude of the airport or, we have to revert to the "Manual" options of the pressurization systems, to make sure the cabin will be properly pressurized when doors are closed, or properly depressurized before doors are open.

Give it a try! 

Roberto

PS: any comments or advice on high altitude operations (that can be added to checklists)  are very welcome!

 

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FCOM Limitations list maximum takeoff and landing altitude as 10000 ft pressure altitude.  You're going to have extensive modifications in place for higher altitudes, the STC will include the required additional operating instructions and placards.

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Hi Dan!

Yes, you remind me of the original B727's that the former local airline, Lloyd Aereo Boliviano (LAB) had, which were fitted with significantly modified engines to fly out of SLLP with apparent ease.

I am not sure of what other modifications they had for other systems, but I am sure there were several.

Recently, I watched a video of an airline called LC Peru, that flew Dash 8-100's    into and out of very high altitude airports in Peru. They did not have modified engines, but they had modified (by their engineering staff)  processes to deal with pressurization, where the PNF would keep his/her hand on pressurization valves/systems and activate the appropriate ones, the moment of touchdown.

Notwithstanding the restrictions you mention, it is still a lot of fun to fly the QOTS II in that region.

Thanks for your comments!

Roberto

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11 hours ago, r_stopnicki said:

Of course! The cabin was at 8000 ft and the guys waiting outside with the equipment, were at 13300 feet!

I manually depressurized the cabin and, presto!, everything worked.

Prior to landing, did you set the landing altitude manually to 8000', Roberto? Just wondering why your cabin was at 8000'.

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Hello John!

To be quite honest, I didn't think much about this issue. I did not touch the pressurization panel.

Alarms have been "normal" in my simulator whenever I took off or landed at SLLP. I understood why they were set off, but did not know much about how to deal with them.

I only paid true attention a couple of days ago, when I wanted to take a screenshot of an opened nose cone/door and couldn't. The EICAS had "8000" in red lettering.

That led to this chain of posts.

I spent my childhood and youth in La Paz.

Already interested in aviation and during the days when they allowed folks into the cockpit, I remember two ocassions (a B727-100 of AeroPeru and a B727-200 of Eastern) where I talked myself into the cockpit for take off, and in both cases one of the pilots and the flight engineer, had oxygen masks on during the turn around.

Further in the "really old" days of Lufthansa driving by SLLP with their B707's, as a passenger in a flight to Santiago, Chile, I noticed a "pop" in my ears as the airplane taxied fro departure. I now understand that obviously, the cabin was being pressurized while still on the ground.

In my "old days" of B767 simulation, somebody provided me with a procedure to manually pressurize or depressurize the cabin at SLLP and I had inserted that into my checklists.

However, that was two home moves ago and several computers ago.

So, I am now reading the FCOM with interest, I am used to checking this forum more than once daily and Mr. Google and I are on a first name basis.

I am also retired, but your credentials are much more impressive. Enjoy a long and healthy one!!

Thank you for your post and interest.

All the best!!

Roberto

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Quote

I only paid true attention a couple of days ago, when I wanted to take a screenshot of an opened nose cone/door and couldn't. The EICAS had "8000" in red lettering.

Interesting, normally warnings and EICAS displays go red only around 10,000 feet. An airline option perhaps? 

I'm currently trying to figure out what should happen if you don't set the landing altitude manually to 8000' (which is the normal procedure for high altitude airport)s. The normal 747-400 is not certified for landings above 10,000 feet, so we are in "uncharted territory".

Normally, the Cabin Pressurisation computer uses the FMC airport altitude or the manually entered landing altitude (on the overhead panel) and the FMC cruise altitude to compute the pressurisation profile for takeoff, climb, cruise, etc. 14,000' can be set manually on the panel, so I assume the automatics can handle an FMC-generated landing altitude of 13,100' (La Paz).

If the landing altitude is greater than 8000', the pressurisation controller will set 8000' as the cabin altitude in cruise. When Top Of Descent is reached (when arriving at an airport higher than 8000'), the pressurisation system will start decreasing the cabin pressure to match the pressure at the airport (plus a small amount, equivalent to 50 feet)). This happens at 1000 feet per minute (which might be hard on your ears). In theory, the cabin would be at approximately 13,050 feet in about 5 minutes after TOD. However, there are two other component in the pressurisation system which will try to stop this happening. At 11,000 feet, these devices will command the cabin pressurisation outflow valves to close (sealing the cabin). If I understand the maintenance manual correctly,  the cabin will stay at 11,000 feet even after landing (Normally on touchdown at airports at lower altitudes, the outflow valves will run to completely open... to equalise the pressure inside and outside the cabin to allow you to open the cabin and cargo doors).

If you turned off the packs and the engines at the gate, the cabin should depressurise (from 11,000 feet to 13,100 feet). The speed of depressurisation depends on a number of factors (Air can escape from the fuselage in lots of different ways)..

1) The Equipment Cooling System reconfigures itself after engine shutdown and if the temperature is above 7C, a valve in the fuselage will open and cabin/cargo air will vent to atmosphere.

2) Flushing toilets will allow air in the cabin to escape.

3) at low cabin differential pressures, door seals are sometimes not so good, so air may leak from them.

4) air may escape via the lavatory and galley sinks.

5) a valve in the fuselage (near the nosewheel) known as the Forward Overboard Valve opens after engine shutdown, allowing air to escape from the cabin

 

Of course, there are a lot more systems which have to be made suitable for landings at La Paz. This is probably why the regular 747-400 is not allowed to land there. As I discovered recently, tyres have to have a special high speed rating. Groundspeeds during landing and takeoff are higher at La Paz than normal.

Anyway, thanks for your stories about La Paz. Very interesting.

I look forward to more reports regarding your landings at La Paz in real life and in PMDG :gaul:

Cheers

JHW

 

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