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787 humidity

Old 31st Aug 2023, 21:22
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787 humidity

I know the 787 has a setting to enter the number of pax and this will affect the humidity introduced by the CACs. However, does setting the number pax to a high number reduce fuel burn?
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Old 31st Aug 2023, 21:38
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BTW the reason I ask my wife is on a 787 and the pilots have set the number of pax to a number far higher than the actual number of pax. This is causing the air to be very dry. When she asked why he did this he said it reduces the fuel burn.
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Old 31st Aug 2023, 22:21
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Setting the passenger count to a low number will reduce the airflow requirement and therefore the electrical load on the cabin air compressors. However there have been problems with the compressors surging, so some airlines choose to artificially inflate the passenger number entered into the cabin attendant panel, improving compressor surge margin but negating this possible fuel saving. There may also be a relationship with humidity, but I am not aware of it.
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Old 31st Aug 2023, 22:29
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I thought a high pax count setting would introduce less humidity from the CACs, more pax = more humidity from pax breathing.

"The chief source of moisture inside these airplanes is passenger respiration." from Boeing website.
"The 787 crew will be able to program the cabin air system for optimal humidity based on the number of passengers aboard" Sinnett the 787 project director.
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Old 1st Sep 2023, 08:52
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Originally Posted by jimvaughan
BTW the reason I ask my wife is on a 787 and the pilots have set the number of pax to a number far higher than the actual number of pax. This is causing the air to be very dry. When she asked why he did this he said it reduces the fuel burn.
With a very low passenger count, humidity will be marginally less with the pax count set to an artificially higher value.
In 5yrs on the aircraft, Iíve never thought of the air as ďdryĒ. Compared to most other jets the low cabin altitude provides a far more comfortable environment on long haul trips.

The fuel saving your wife was quoted is rubbish. We will see a slightly higher fuel burn for a higher pax number entered.


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Old 1st Sep 2023, 12:56
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The original purpose of the passenger count was to reduce CAC energy consumption and consequently fuel burn, in addition to providing a more comfortable environment. Unfortunately the CACs donít like operating at this reduced power setting and have a tendency to surge and shut down. The solution is to run them at full power, which limits the surging.

This does tend to result in quite a dry cabin environment but itís not being done for fuel saving reasons and has the opposite effect.
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Old 1st Sep 2023, 19:12
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Thank you, and yes, the cabin is indeed noticeably drier.
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