band of the glideslope signal. The reception of such a signal could result in the movement of the glideslope receiver needle and present a false indication to the pilot.
1.16.3.1 Guam Instrument Landing System and Potential Interference From Spurious Radio Signals
An FAA National Resource Engineer for Navigation testified at the Safety Board's public hearing about the Guam ILS system and the potential for interference from spurious radio signals. The engineer stated that "the pilot would normally be warned that a signal is not present by the presence of a flag, a warning flag, that indicates that something about the receiver system or something about the ground system is abnormal...." The engineer testified that he assumed that the accident flight crew's remarks regarding the glideslope (as recorded on the CVR) had to do with the presence or absence of flags. He concluded that "...there must have been some sort of flag activity coming into view, disappearing from view, some time during the approach" and that the comments, although they did not convey information about the duration of any flag activity, indicated that "...there must have been enough absence of the flag for the crew to occasionally decide that the system was on the air when in fact it wasn't...."
The FAA engineer also testified that, although the glideslope at Guam International Airport had been removed from its shelter, radio signals generated by some other source could have provided an intermittent signal to the glideslope receiver, which might have prevented the instrument warning flag from remaining in view. The engineer explained that potential external sources of noise and unintended signals, which are normally too weak to be heard, can be heard on an empty channel and that, during airborne flight tests of ILSs in which the localizer or the glideslope is turned off, it has been fairly common for the cockpit instrumentation to record intermittent indications of flag and needle activity. However, he expressed that this sort of activity on the instrumentation (referred to by pilots as "flag pops") is typically intermittent and of very short duration.
The engineer testified about the types of radio signals that could potentially cause a movement of the flag. He stated that the ILS transmits two tones and that the difference in the signal strength of the tones deflects the glideslope fly-up and fly-down needle. The engineer indicated that the receiver has some circuits that look for these two tones and that the fly-up/fly-down needle indicates the difference in strength of those two tones. He added that "...the difference will be zero, and the needle will be centered when the two tones are equal...."
Further, the FAA engineer stated that the flag circuit, the other indication that a pilot sees, is driven by a signal that is the sum of the two circuits or the two signals. He indicated that "as long as the 90 and 150 [Hz] signals are both present at sufficient strength, the flag will remain out of view." The engineer also stated that, if there is no ground station transmitting and no intended ground station but some other signal, then those portions of the signal that contain 90- and 150-Hz tones[1] would still get through those filters and could cause the needles to deflect. The engineer added that, depending on
- ↑ The FAA engineer indicated during his testimony that voices and music contain 90- and 150-Hz components.
