Excellent work Mark!

How did we approach this problem???

Pulled the plug behind the test switch panel. Grounded the wire going to card. Received both the overhead annunciator and EICAS message. (Card working properly)

Shot switch on the panel itself. (Switch working properly)

Shot each end of loop from panel plug. (15 Meg Ohms - Open loop)

Instead of dropping the underside leading edge panels to expose the loop connections themselves, the switch area was looked at first. One quick visual of the terminal in question revealed what the problem was. It was verified by leaving both terminals off the switch. A jumper to ground on the good terminal wire gave the warning. No warning was seen on the bad terminal wire. Some insulation was scraped through and the jumper attached..... received the warning.

New terminal installed............. Problem solved.

767-200 with a right wing duct overheat system that would not test.







The duct overheat systems (Left, Right, Body) use detector loops and/or overheat switches that provide a ground signal in an overheat condition.



During a system test, integrity is verified by providing a ground signal that must pass through the entire length of loops and wires.



Signal path during test.




This aircraft had a broken wire on a terminal at one of the overheat switches in the pylon. The switch(s) only use two connections. A single wire hooked to ground and a double wire connection used in the loop path. If the single wire was broke, the system would test normally, but that switch would have no ground path for warning if it happened to overheat. The broken wire was on the double wire connection. With the signal path open, a proper test could not be confirmed.





The PRSOV (Pressure Regulating and Shut-off Valve) is located in this area. It is VERY difficult to remove this valve and it is almost certain that the terminal in question was broken during a valve change.

A note about the functionality of these systems: Even with a broken loop path wire, in an overheat condition, a warning will still be shown. Each end of the loops are connected together at the test switch. The card only needs to see a ground. That ground can come from either side of a broken loop system. This same theory is used on engine fire systems.




767-200 WDM 26-18-21

That's great info I will remember that one, thanks Mark!!

767-200 with the ILS (Instrument Landing System) function of the standby horizon inoperative. The deviation bars along with associated flags in view no matter what position control switch was in.



The ILS indication is controlled via the Static Inverter/ILS Processor unit located on the E1-4 shelf. This unit is located next to the Center Multi-Mode Receiver. (The MMR's have two functions, GPS and ILS.)



The Center MMR feeds ILS information to both the Static Inverter/ILS Processor and the Center EFIS SGU with a single 429 data bus output.

With the indicator switch on the "Off" position, both bars and both flags should be biased out of view. In the "ILS" position with a valid ILS signal, the flags should be out of view and the bars should indicate actual left/right-up/down information.

To verify the operation of the Center MMR, either the Captain's or F/O's EFIS switch can be placed in the "Alternate" position and a test run on the ILS Control Panel. (The test should display "up/left then down/right. The EADI mode select should be placed into "Approach" for this test to show.)



In the ILS position, the Standby Horizon deviation bars should mimic the bars on the EADI selected to the Alternate EFIS SGU.

In the case of this particular problem, the issue turned out to be the Static Inverter/ILS Processor Unit itself. All outputs had failed.

The Standby Horizon does not control any functions for the indication. Flag voltage and bar deviation signals all come from the Inverter/Processor unit. Having all four failed in the "Off" position would tend to say the box is bad vs the indicator.





767-200 SSM 34-24-01
AWM 34-24-11