ATA 26-16, Lower Cargo Smoke Detection

Quite a few 767's that have been converted to freighters have a modified fwd and aft cargo smoke detector system.
  
     
  
Each cargo hold has two detectors. Air is drawn into the detectors from lines (orange in this case) that begin near the top of the compartment in several locations. Two blower motors (although they're really sucking) are located below the detectors and the (negative) pressure switch (gold colored disk in image above).
  
     
  
Only one blower is used at a time. If the switch senses a lack of vacuum (or lack of air flow) for 60 seconds, it will activate the secondary blower motor. A EICAS message will be generated to indicate the failure.

The outlet of the blower motors are located on a common mount plate.
  
     
     
  
On the back side of this plate opening is a thin rubber flapper. Because they are connected to the same basic "plenum", while one blower is operating, the flapper on the other motor is pulled shut.
  
     
  
As we recently found out..... if the flapper of the idle blower fails to close (falls off, has holes, etc...) air will be drawn through the opening to the operating blower. There will be no air drawn through the detectors and the pressure switch will cause an EICAS message.

Further explanation.....

The square opening at back of the operating blower is used for discharged air.

When a blower is running, its own flapper is being pushed back to let the air escape.

The non-running blower will have its flap pulled "in" because of the suction through the manifold.

We recently had a aircraft with a #1 blower failure (the #1 is the primary and is powered up first).
The blower failed which caused the system to use the #2.
The flapper on the back of the #1 was missing.
As the #2 was blowing, air entered the back of the #1 blower into the manifold.
No air was drawn through the detectors and the pressure switch was not being activated.
The system threw a EICAS message of "Cargo Det Air" because all smoke detection in that bay was inoperative.
  
Crappy image, but  it shows the location of missing flap for the #1 blower.
  
     
  
Replacement flapper.
  
     

 

Worked on these quite a few times never knew about the flapper, always suprised me how much smoke is actually required to set one of these things off.

You hope the thing is there.
On this latest aircraft, our sheet-metal guy was able to replace the forward one without removing the whole detector/sensor/fan mount assembly.
A one hour change could of changed to a whole day affair.