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EmileB

Field elevation setting

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I have searched the FCOM, FCTM and this forum but, unless I overlooked something, did not find an answer to the following :

The landing field elevation on the pressurisation panel is set in 50ft increments. What do I set it to for "in-between" values ? For example, the elevation of my hometown airport NZAA is 23 ft. Would I set the field elevation to 0 ft or to 50 ft on inbound flights ? Thanks for any info.

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You can set it to 0 in that case. If you look for ex. at OMDB Dubai charts ILS RWY 12R, the airport elevation is 59 ft, but with alt. set hPa the Rwy elevation is 0 hPa. With ILS 30R Rwy, the landing altitude hPa will be 2ft, but you cannot select it, so 0 again is good. Most quickly way is to read allways the airport destination chart (of course making referiment to the runaway wich you land).

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You can set it to 0 in that case. If you look for ex. at OMDB Dubai charts ILS RWY 12R, the airport elevation is 59 ft, but with alt. set hPa the Rwy elevation is 0 hPa. With ILS 30R Rwy, the landing altitude hPa will be 2ft, but you cannot select it, so 0 again is good. Most quickly way is to read allways the airport destination chart (of course making referiment to the runaway wich you land).

I may have misunderstood you but if you have the altimeters set for QNH then the landing field elevation at OMDB 12R will be 59 ft. Best to choose 50 in that case. However if you set QFE then the landing elevation would always be zero.

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I have searched the FCOM, FCTM and this forum but, unless I overlooked something, did not find an answer to the following :

The landing field elevation on the pressurisation panel is set in 50ft increments. What do I set it to for "in-between" values ? For example, the elevation of my hometown airport NZAA is 23 ft. Would I set the field elevation to 0 ft or to 50 ft on inbound flights ? Thanks for any info.

 

The altitude is usually set to the next higher altitude in GA aircraft (100-500' above, depending on the unit/aircraft/etc). NZAA being 23' MSL, set the pressurization panel to 50' (using NGX numbers, here). The idea is that, when descending through 50', the pressurization system will seek equilibrium by opening the valves prior to touchdown.

 

On the other hand, the 777's system will automatically set an altitude intentionally below the landing field elevation to maintain positive pressurization until the weight on wheels indication (I'm guessing for structural rigidity), where the pressure will be removed by opening the outflow valves. From what I'm reading of Boeing aircraft, despite having to set the elevation manually, the AUTO mode of pressurization seems to always force slight pressurization by setting the landing alt artificially to 200-300 below the set/stored altitude. Knowing this, it seems that setting it to whatever is closest will work, though I recall seeing an airline SOP that follows the "next highest" value setting, just for consistency.

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On the other hand, the 777's system will automatically set an altitude intentionally below the landing field elevation to maintain positive pressurization until the weight on wheels indication (I'm guessing for structural rigidity), where the pressure will be removed by opening the outflow valves. From what I'm reading of Boeing aircraft, despite having to set the elevation manually, the AUTO mode of pressurization seems to always force slight pressurization by setting the landing alt artificially to 200-300 below the set/stored altitude. Knowing this, it seems that setting it to whatever is closest will work, though I recall seeing an airline SOP that follows the "next highest" value setting, just for consistency.

 

I think I read that the positive pressure is to minimize pressure 'surge' when applying takeoff or reverse thrust power.  Not sure where I read that but it stuck in my mind, which highlights the randomness of my retention ability.

 

I always set the cabin to something a little higher than field elevation in the NGX or the Cessna Chancellor.  Realistically, the dumps open on squat so you're just trying to get in the ballpark.  It takes several hundreds of feet to make a noticeable difference in your ear.

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I'm guessing for structural rigidity

The fuselage structure is very rigid already. A slight pre-pressurisation (about 0.1 psi) won't make any difference. Also what about the wings? They aren't pressurised but need to be even more strong. I think this commonly held idea comes from the way a tin can is stiffer when pressurised than when not. But a tin can doesn't have any internal reinforcement structure.

 

As Dan said, the pre-pressurisation is to smooth out cabin pressure bumps. If pressurisation started at lift off there would be a sudden movement of the outflow valve from full open to nearly full closed. If you do this on ground before takeoff it can all be done slower and more smoothly.

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Worse case scenario it's only a 25 foot difference.  Just round it to the nearest value and don't worry about it.

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I may have misunderstood you but if you have the altimeters set for QNH then the landing field elevation at OMDB 12R will be 59 ft. Best to choose 50 in that case. However if you set QFE then the landing elevation would always be zero.

Yes I put the max. field elevation on that airport. The correct values are:

rwy30L - 59ft

rwy30R - 31ft

rwy12R - 10ft

rwy12L - 10ft

 

Infact the landing elevation with altimeter set hPa is allways zero on all runaways with no autoland. But if I use autoland with radio altimeter on 30L then the landing elevation is 50ft. Correct me if I'm wrong.

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But what if it's a leap year and an El Niño?

 

Seriously, look at the chart.

Round the field elevation to the 50 ft closest.

That's it.

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Thanks for the helpful advice, everyone !  I now know what to do.

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Yes I put the max. field elevation on that airport. The correct values are:

rwy30L - 59ft

rwy30R - 31ft

rwy12R - 10ft

rwy12L - 10ft

 

Infact the landing elevation with altimeter set hPa is allways zero on all runaways with no autoland. But if I use autoland with radio altimeter on 30L then the landing elevation is 50ft. Correct me if I'm wrong.

Autoland has nothing to do with cabin pressurisation settings. The cabin landing altitude will be set exactly the same with a manual landing. I think you might be getting the OP's question confused with the radio/baro minimums selection. Even so you aren't correct. You must set the decision height either as radio altitude (rad) or as a pressure altitude (baro). It'll never be zero. The chart tells you what the appropriate setting is for each runway.

 

You can set the altimeters to QNH, QFE or STD, "altimeters set hPa" is meaningless. hPa or inHg is determined by whether you are using metric or US units.

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You guys are putting way too much thought into this.  Just set the landing elevation to the destination field elevation to the nearest 50 feet and forget about it.

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