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BuildsJets here with your aviation trivia. Cabin Air Quality requirements generally have become a lot more stringent since they were first enacted back in Dece

BuildsJets here with your aviation trivia. Cabin Air Quality requirements generally have become a lot more stringent since they were first enacted back in December 1964. Previously, only the crew was required to be supplied with fresh air, the rest of the cabin was required to be "ventilated" but no ventilation rate was specified, so it was frequently turned off to save fuel or increase power. (This is approved and is still done on some older generation aircraft that are still in service, for instance the Boeing 737NG for certain high performance situations, search 737 Packs Off Takeoff) The current requirement is .55 lbs of outside air per minute per passenger minimum, through all phases of flight. Permissible CO2 has been reduced from 3% in 1964 to 0.5%. today. The carbon monoxide requirement is unchanged at 1 part per 20,000. Multiple other requirement changes, but those are the ones that are specific and measurable. Current requirements are published in 14 CFR 25.831: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-25/subpart-D/subject-group-ECFR99abb8c7d4bf678/section-25.831 https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C... Original requirements are on page 47 of this PDF: https://archives.federalregister.gov/issue_slice/1964/12/24/18276-18353.pdf#page=16 https://archives.federalregister.gov/issue_slice/1964/12/24/...

wkat4242 4y ago
> Previously, only the crew was required to be supplied with fresh air, the rest of the cabin was required to be "ventilated" but no ventilation rate was specified, so it was frequently turned off to save fuel or increase power. (This is approved and is still done on some older generation aircraft that are still in service, for instance the Boeing 737NG for certain high performance situations, search 737 Packs Off Takeoff) How does that work though? The cabin pressurization requires constant bleed air from the engines because there are intentional leaks. So the air is constantly renewed anyway.
- buildsjets 4y ago
  The cabin air pressure requirement (psi) is more or less a constant, but the cabin air flow rate requirement (lbm/min) is a variable which depends on the number of passengers on board, the air temperature conditioning requirements, the cargo being carried, the altitude (outside air pressure), and the age/condition of the aircraft. (Things like rubber door seals are not perfect and develop leakage rates over time. Sometimes you can even hear it if you are near the door.) The engine pressure regulating/shutoff valve and the cabin outflow valve position are regulated by the pilot (legacy aircraft) or by the digital cabin pressure control system (current aircraft) to maintain the required flow and pressure (cabin altitude setting). In ye olden days before a DCPCS, not only were the flow requirements not as stringent, but there was culture of "cheating the chart" when manually setting the system. More airflow = more fuel burn, and at one time pilots were compensated for minimizing fuel burn. Anyone can see that this is a perverse incentive from a safety standpoint, just look at what former astronaut Hoot Gibson did to the passengers on his 727 trying to save a buck and playing flight test pilot with paying passengers onboard. https://en.wikipedia.org/wiki/TWA_Flight_841_(1979) https://en.wikipedia.org/wiki/TWA_Flight_841_(1979) Also, from a cabin crew perspective, drowsy passengers are conveniently compliant passengers.

wkat4242 4y ago

> Previously, only the crew was required to be supplied with fresh air, the rest of the cabin was required to be "ventilated" but no ventilation rate was specified, so it was frequently turned off to save fuel or increase power. (This is approved and is still done on some older generation aircraft that are still in service, for instance the Boeing 737NG for certain high performance situations, search 737 Packs Off Takeoff) How does that work though? The cabin pressurization requires constant bleed air from the engines because there are intentional leaks. So the air is constantly renewed anyway.

buildsjets 4y ago
The cabin air pressure requirement (psi) is more or less a constant, but the cabin air flow rate requirement (lbm/min) is a variable which depends on the number of passengers on board, the air temperature conditioning requirements, the cargo being carried, the altitude (outside air pressure), and the age/condition of the aircraft. (Things like rubber door seals are not perfect and develop leakage rates over time. Sometimes you can even hear it if you are near the door.) The engine pressure regulating/shutoff valve and the cabin outflow valve position are regulated by the pilot (legacy aircraft) or by the digital cabin pressure control system (current aircraft) to maintain the required flow and pressure (cabin altitude setting). In ye olden days before a DCPCS, not only were the flow requirements not as stringent, but there was culture of "cheating the chart" when manually setting the system. More airflow = more fuel burn, and at one time pilots were compensated for minimizing fuel burn. Anyone can see that this is a perverse incentive from a safety standpoint, just look at what former astronaut Hoot Gibson did to the passengers on his 727 trying to save a buck and playing flight test pilot with paying passengers onboard. https://en.wikipedia.org/wiki/TWA_Flight_841_(1979) https://en.wikipedia.org/wiki/TWA_Flight_841_(1979) Also, from a cabin crew perspective, drowsy passengers are conveniently compliant passengers.