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Question about MD80 APU

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Well-known member
Nov 26, 2001
Does the APU on the MD80 provide bleed air to the cabin air system? Or does it have an electrically powered air conditioner on the ground? Just researching that case where Boeing and Honeywell got sued by some FA's saying that the air provided by the APU was bad. I didn't know that APUs could provide bleed air to the cabin air system.
APU's do provide bleed air to operate the aircraft environmental system, as well as air for the pneumatic driven engine starter. APU bleed is not permitted in flight. Electric power from the APU is available on the ground and in flight with a few minor limitations.

The APU is located in the tail of the aircraft and provides conditioned air to the cabin while in the air or on the ground (can't remember the limitations for in-flight or not). The inlet/scoop for this unit is located on the bottom of the fuselage...at the point where the fuselage curves up the tail.

With regards to the FA deal, hydraulic fluid from the main gear area, in-flight, would stream down the belly and into the scoop and thus contaminate the conditioned air making for a very sick flight crew, and pax's, but more so FA's 'cause they were exposed for longer periods, like all day.

The problem the FA's are having now (law suit) is trying to get the pilots to come aboard with them, which they won't do...medical reasons. The FA's claim neurological damage from the bad conditioned air, pilots won't go anywhere near that of course.

I'm sure someone else could go deeper with the FA's deal...hope this helps.

Last edited:
Only on the ground

I'm not familiar with the details of the FA's case, but if RJones is correct, It's no wonder that they lost.

Here's why I say that. First, the APU can only be used to supply air to the airconditioning pacs when on the ground. Second the APU is mounted in the tail compartment and the Hydraulics are concentrated in the wheel wells(about 30 ft forward). It seems obvious to me that no hydraulic fluid will stream back during the time that the APU is providing A/C air because there is no front to back airflow when the airplane is sitting still at the gate.

This seems too obvious, maybe I'm missing something.

The MD-80 APU is located in the lower aft fuselage, however the intake in the bottom of the aft fuselage is for combustion air only. The air for cabin heating and cooling comes from the ram air scoop on top of the aircraft at the lower forward vertical stabilizer in flight. On the ground the environmental systems air comes from the ram air scoop (left system) or the aft fuselage just behind the aft pressure bulkhead. (right system)

There would have to be a large amount of contaminates in the intake area to effect the air quality in the cabin after the air has been thru the aircycle machine. Air contamination on the ground is improbable and in flight nearly impossible.

Use of APU air is prohibited in flight by the AFM. The APU may be used in flight as an additional source of electrical power during most phases of flight with only a few minor limitations.

I stand corrected with repsect to the APU in-flight, can't remember much if anything about the MD-80. Below you'll find a link that more accurately outlines the problem. From what I gather this law suit is still on-going.


Also: http://www.afanet.org/aq_bulletin.htm

"Even though the airlines are not required to monitor the air supply, the FAA has formally acknowledged that the air supply, at least on certain aircraft types, can get contaminated in this way. On 8 August 2000, they published an Airworthiness Directive that requires certain modifications to the hydraulic lines and related parts in the APUs of certain McDonnell Douglas aircraft types. The directive was "prompted by reports of smoke and odor in the passenger cabin and cockpit due to hydraulic fluid leaking into the APU inlet, and subsequently, into the air conditioning system" (65 FR 48368, August 8, 2000). This is a step in the right direction but will only address a very small piece of the problem."

My wife informs that up to 8 quarts of hydraulic fluid leaked into the APU. With respect to the pilot's...they aren't going anywhere near this 'cause of medical certification reasons...I don't blame them, maybe when they retire.


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