Mark

It's winter, cold has $hit, sideways blowing snow and sleet the previous day and you walk up on to an aircraft and see flags on the airspeed and altimeter indicators.
  
     
  
You could say..... bad Air Data and swap them, but the problem stays on the original side???
  
     
  
Now what?

From previous experience, I know that this is a clogged pitot probe. I'm guessing (but I believe I'm right) that once the ice clog sealed off the pitot system, outside barometric air pressure went up. The Air Data now is sensing this higher pressure static air. So...... the Air Data expects equal pitot/static pressures on the ground and always higher pitot pressure than static while flying.

What do we have in the case of a fixed (clogged) pitot pressure that is lower than the now higher static pressure...... negative airspeed. The air pressure inputs are out of tolerance as far as the Air Data computes them, hence..... flags!!!

The first thing to try (which usually corrects the problem) is to just test pitot heat. The probe gets hot, melts the ice plug, pressures equalize, and the flags pull out of view.
  
     
  
Not in this latest case of ours. We did have water/ice in the probe, but we also had it in the pitot hose inboard from the probe. This is "not" a normal situation unless some dummy (me) put the Pitot/Static test box on it and blew the ice/slush plug into the hose by applying test box pitot pressure.

Either way, we took off the probe and used compressed air blowing from the Air Data hose outward to the probe quick disconnect.
  
     
     
  
What really threw us at the start was the fact that I applied 300 knots of airspeed to the system early on. When I dropped the test box back to 0 knots..... the airspeed indicator stayed high? Now I've got pressure that made it pass the ice blockage in the hose or probe, but it didn't fall when the test box was lowered. In other words, I've got pressure stuck in the hose.

The hose quick disconnects are spring loaded closed when removed from the Air Data (and in this case , the probe. Not all probes have quick disconnects). The Air Data side of the connection is open to ambient. I filmed the airspeed indicator as a mechanic disconnected the pitot line from the ADC. (Video below) 
  
As a note; during the air purge of the system, both quick disconnect springs had to be overridden to allow air to pass through the hose.
  
 

On 767's, there are two UFI (Universal Fault Interrupter) units that protect the center tank pumps and wiring.

We recently had a right pump that was only receiving one (out of three) 115VAC inputs. A control relay upstream of the pump had only one phase also.

Traveling further upstream is the UFI. There were some fault lights on the unit indicating a phase problem.

So, you get a replacement and hang it off the plug (thinking it'll work). No lights..... no nothing?

Hang it off the known good side..... same thing?

The only possible issue would be case grounding, which it turned out to be. Most all aircraft electrical components have a wired ground. Not the UFI's.

Not really a big deal, but it sure makes you think for a second.
  
     
     

I'm not going to be of much help, but have you looked at the aircraft side wiring?

Twisted pair shorts, either to each other, to shield, or to ground.
High resistance on any wires from ECU to APU. Especially look at the speed sensor wiring.

You seemed to have covered all the APU related items quite thoroughly.

Section 49-00-00-810-801 of the manual you sent me shows all the failure codes that can be read.

I also found this in the manual.....
  
     
 

I'm somewhat confused.
You pulled a working APU FADEC (or ECU - Electronic Control Unit) from the aircraft in question and installed it on another aircraft. Now you have issues.
Did you try putting the original ECU back in?

RPM is a fuel control issue. You stated you changed it. RPM's are fluctuating.

I'm I to assume it has a DC generator installed? If it's an AC generator, you can verify RPM's by watching the 400Hz output. If it's climbing and diving along with APU speed..... you still have a fuel control issue.

You swapped the harness, so rules out wiring.

Any other faults showing up on the FADEC?

From member Emblem DHC.....


Currently on OUR Embraer legacy 600 we are facing a serious problem with APU ( APU SHUTDOWN AUTOMATICALLY)

Aircraft is serviceable it's APU FADEC Is removed for another aircraft and when we installed new APU FADEC APU start shutdown automatically after 1 hour of operation
So we check APU FADEC FAULT Message
1. SPEED SENSOR FAULT ( AS per this SPEED SENSOR CHANGED)
2. SYSTEM B/U OVSPD ( BACKUP OVERSPEED) AS per this  FCA Fuel control assembly changed , fuel soleniod valve , APU Engine harness swapped)
But actual there is no OVERSPEED

Now problem becomes worse APU Start shutdown after 3-10 mintues,
Sometime in start cycle , sometimes work for 1 hour

One thing we notice RPM is increasing slowly after reaching 95
It's slow down  its like 96, 97then back to 96 and then increase to 100

Part number of individual switch.....
P/N 141SM2T2
IPC 22-32-01-50
     
     

 

Let's look at the pressurization problem....

Manual mode is 28VDC powered and operated completely by the overhead control panel. It uses a DC motor on the outflow valve.
This "is not" a preferred mode of operation as it takes considerable effort to maintain proper pressurization through all phases of flight.

In Auto mode, the system is controlled by the Pressurization Transducer. It is a AC system. Inputs are (either) ADC, Air/Ground, and the Landing/Cabin Alt Selection Panel (on right side panel). The outflow valve motor is AC powered.

What exactly is the crew saying as far as the fault goes?

Has power been checked? Any fault lights on transducer? Has the alternate Air Data been tried? Is the outflow valve reacting when simulating air mode? So many more questions.....
  
     
 

I have saved both PDF's.

Because of copyright infringement, I cannot have complete manufacturer data displayed on the site.

They can be sent to other users upon request.

I will look at both to see if I can be of any help to you.

I don't have any reference material for either issue. Can you send me a schematic or wiring diagram for each system. A training manual would be helpful also.