This accident shows the importance of reporting unusual observations to ATC.
The snow buildup had affected the localizer signal. This does definitely happen, to the glideslope too. A crew had noticed it and reported it to ATC. However, as I understand it from my time in ATC, ATC (by policy) needs a second, confirming report before they take any action. With only one report, it could very well be the aircraft instruments that are off. But with two reports, it is almost certainly the system itself. Unfortunately, while other airplanes saw the same thing, they did not report it.
If ATC had received a second report, the system would almost certainly have been NOTAMed OTS. A Flight Check crew may have been sent up to evaluate it, depending on whether the ground techs were able to confirm the problem or not.
Still that way.
2−1−10. NAVAID MALFUNCTIONS
a. When an aircraft reports a ground−based NAVAID malfunction, take the following actions:
1. Request a report from a second aircraft.
2. If the second aircraft reports normal operations, continue use and inform the first aircraft. Record the incident on FAA Form 7230−4 or appropriate military form.
3. If the second aircraft confirms the malfunction or in the absence of a second aircraft report, activate the standby equipment or request the monitor facility to activate.
4. If normal operation is reported after the standby equipment is activated, continue use, record the incident on FAA Form 7230−4 or appropriate military form, and notify technical operations personnel (the Systems Engineer of the ARTCC when an en route aid is involved).
5. If continued malfunction is reported after the standby equipment is activated or the standby equipment cannot be activated, inform technical operations personnel and request advice on whether or not the aid should be shut down. In the absence of a second aircraft report, advise the technical operations personnel of the time of the initial aircraft report and the estimated time a second aircraft report could be obtained.
b. When an aircraft reports a GPS or WAAS anomaly, request the following information and/or take the following actions:
1. Record the following minimum information:
(a) Aircraft make, model, and call sign.
(b) Location or position, and altitude at the time where GPS or WAAS anomaly was observed.
(c) Date/time of occurrence.
2. Request a report from a second aircraft.
3. Record the incident on FAA Form 7230−4 or appropriate military form.
4. Inform other aircraft of the anomaly as specified in subparagraph 4−8−1k, l, or m, as applicable.
PHRASEOLOGY−
ATTENTION ALL AIRCRAFT, GPS REPORTED UNRELIABLE (OR WAAS UNAVAILABLE) IN VICINITY/AREA (position).
EXAMPLE−
“Attention all aircraft, GPS reported unreliable (or WAAS unavailable) in the area 30 miles south of Waco VOR.”
c. When a pilot reports a WAAS anomaly, determine from the pilot what indications he or she observes and record the information in accordance with subparagraph b above.
