One dark afternoon, flying into my home base of Cincinnati, I set up for an approach to runway 27. As part of the pre-landing check list my co-pilot pulled a small knob labeled ‘Test’ located on the navigation instrument used to shoot the approach.There were two navigation meters on the instrument; one designed to show alignment with the localizer and one that showed alignment with the glide slope.
(The localizer signal helps the pilot line up laterally with the runway and the glide slope signal provides vertical guidance to the runway. In instrument flight conditions, the two signals bring the aircraft down to 200 feet above the ground for non-airline operations.)
When the co-pilot pulled on the test knob, the localizer meter movement was driven horizontally to the extreme left position, and the glide slope meter was driven vertically to the top of its travel. Releasing the knob, the meters returned to their centered positions. This completed the test and the check list item was dutifully cleared.
We intercepted the localizer signal and aligned ourselves laterally with the runway. I remember thinking it was strange that the vertical guidance meter had remained centered as we lined up laterally with the runway. But no matter, it must be OK since the test mode had verified correct operation.
I reduced power and down the glide slope we went. Or so it appeared.
We were monitoring the altimeters as well as the navigation instruments and becoming increasingly concerned. Another instrument (called the DME - Distance Measuring Equipment), showed that the airplane was considerably farther from the end of the runway than it should have been for the altitude, yet the glide slope meter remained centered.
Whereas before I had congratulated myself on nailing the glide slope, I was now wondering what was going on. The co-pilot and I decided that we weren’t going any lower then allowed for an approach without a glide slope.
We broke out of the clouds a bit over 1/2 mile from the end of the runway at a low altitude with the glide slope meter still showing we were properly positioned vertically. The approach was safe because we hadn’t descended below the “localizer only” minimums. We didn’t kill our passengers or roll the airplane up in a ball of aluminum. But what had happened and why?
A few days later I visited the local avionics shop and asked to look at the maintenance manual for the navigation instrument.
It had two receivers, one for the localizer and another for the glide slope. The outputs of the detectors in the receivers were rectified AC signals which were coupled to Op Amps that drove the respective meters. If the aircraft was properly positioned on the localizer and glide slope, the output from each Op Amp would be zero volts and the meters would be centered.
The test circuit applied a fixed DC voltage via isolation diodes and dropping resistors on each meter’s Op Amp input. That was the extent of the test circuit.
The technician found that the Glide Slope receiver was dead - no output - which explained why the meter stayed centered. Not knowing what the test mode did on the day of the incident, I had assumed everything was wonderful since the test function worked as expected. I didn’t know that the receivers were not checked in the test mode.
Cost constraints probably drove this design decision but from a pilot’s perspective, you have to question the value of a test mode with such limited functionality.
As it turns out, this design approach isn’t all that unusual in avionics. Most communication radios have a test mode which merely opens the squelch. But it isn’t called Squelch Test. It’s called Test.
A device known as a Marker Beacon Receiver has a test mode that simply lights three light bulbs - bulbs that normally illuminate as you pass specific locations on an instrument approach. The receiver has an audio output that indicates the same events with tones but it’s not tested. And the receiver’s 75 MHz signal detection ability isn’t tested at all.
Annunciator panels have a test function too. But the function usually just tests the light bulbs in the panel, not the functionality of the associated alarm circuitry.
One more example: A heater on one series of aircraft I fly has a press-to-test function labeled “Overheat - Press-to-Test”. Until I became more curious about airborne test functions and started doing some research, I assumed it tested the heater’s overheat sensor functionality. But that would be wrong. Once again, it tested the light bulb that illuminates if an overheat occurs — nothing more.
If a test mode isn’t really a test mode, shouldn’t there be a section to explain actual functionality somewhere in the operation manual? And shouldn’t the device itself be clearly labeled so that you know you’re only checking meter movements or light bulbs and not the unit’s full capabilities?
After all - there’s no pause button in an airplane like there is in the Microsoft® Flight Simulator program. In a real airplane, events are happening pretty fast and you can’t freeze the program while you sort things out.
John Loughmiller is an Electrical Engineer, Commercial Pilot, Flight Instructor and a Lead Safety Team Representative for the FAA.