An small aircraft pilot followed a faulty GS signal until ATC gave him a low altitude alert.

Attributes
ACN	266705
Time
Date	199403
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : elm
State Reference	NY
Altitude	msl bound lower : 2400 msl bound upper : 3300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : elm tower : sua
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Navigation In Use	ils localizer & glide slope : ielm
Flight Phase	descent : approach descent other
Route In Use	approach : straight in
Flight Plan	IFR
Person 1
Affiliation	Other
Function	instruction : instructor oversight : pic
Qualification	pilot : instrument pilot : cfi
Experience	flight time last 90 days : 100 flight time total : 700
ASRS Report	266705
Person 2
Affiliation	Other
Function	flight crew : single pilot instruction : trainee
Qualification	pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe non adherence : published procedure other anomaly other other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While flying with an instrument rated pilot inbound on the ILS 24 into elmers, the GS indicator came alive and the pilot proceeded to follow it. After a short time period, the off flag on the instrument went off but the pilot did not correct for in. After continuing a shallow descent, ATC advised the pilot of low altitude. 2400 ft when should have been 3300 ft for vectoring. The pilot climbed back to MVA and continued the approach, and several other practice approachs. In my opinion, the cause of the problem was the pilot's improper scan. By not noticing the GS flag and not relying on other instruments in the aircraft, the pilot proceeded to fly below the GS. We were in VMC and I was closely monitoring the terrain. To reinforce the importance of a proper scan, I allowed ATC to notify the pilot of low altitude which was followed by additional flight and ground instructions.

Original NASA ASRS Text

Title: AN SMA PLT FOLLOWED A FAULTY GS SIGNAL UNTIL ATC GAVE HIM A LOW ALT ALERT.

Narrative: WHILE FLYING WITH AN INST RATED PLT INBOUND ON THE ILS 24 INTO ELMERS, THE GS INDICATOR CAME ALIVE AND THE PLT PROCEEDED TO FOLLOW IT. AFTER A SHORT TIME PERIOD, THE OFF FLAG ON THE INST WENT OFF BUT THE PLT DID NOT CORRECT FOR IN. AFTER CONTINUING A SHALLOW DSCNT, ATC ADVISED THE PLT OF LOW ALT. 2400 FT WHEN SHOULD HAVE BEEN 3300 FT FOR VECTORING. THE PLT CLBED BACK TO MVA AND CONTINUED THE APCH, AND SEVERAL OTHER PRACTICE APCHS. IN MY OPINION, THE CAUSE OF THE PROB WAS THE PLT'S IMPROPER SCAN. BY NOT NOTICING THE GS FLAG AND NOT RELYING ON OTHER INSTS IN THE ACFT, THE PLT PROCEEDED TO FLY BELOW THE GS. WE WERE IN VMC AND I WAS CLOSELY MONITORING THE TERRAIN. TO REINFORCE THE IMPORTANCE OF A PROPER SCAN, I ALLOWED ATC TO NOTIFY THE PLT OF LOW ALT WHICH WAS FOLLOWED BY ADDITIONAL FLT AND GND INSTRUCTIONS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.