Narrative:

Aircraft was on ILS approach to bur between budde (OM) and vinne (LMM), when bur tower called 'altitude alert, pull up 400' immediately.' this instruction was followed even though ILS G/south displayed fly down. Airplane altitude was approximately 2300'. Tower was so advised. 1bur109.5 was selected on #2 navigation receiver and G/south display agreed with #1. After pull up, both G/south displays showed maximum fly down signal, and when runway was sighted, it was apparent that airplane was too high as LMM was approached, flaps were extended from approach to landing and propellers windmilled to increased rate of descent. A normal landing was made, slightly long. During a flight the previous day and on an ILS approach following the reported approach, the ILS G/south display indicated normally. There was no safety of flight involved in the reported occurrence, but under other conditions, there could be. This occurrence was discussed with an experienced and knowledgeable FAA employee and the following possible conditions for this occurrence are suggested: 1) electronic radio frequency interference radiated from the ground. 2) the altitude alert signal to the controller may, under certain conditions, be generated by a computed value, based on rate of change of altitude, speed (distance), rather than an actual instantaneous airplane altitude.

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Original NASA ASRS Text

Title: TWR RECEIVED LOW ALT ALERT ON GA SMT ON ILS APCH AND CALLED OUT PULL UP. LATER DETERMINED GLIDE SLOPE WAS MALFUNCTION OR ALERT WAS FALSE.

Narrative: ACFT WAS ON ILS APCH TO BUR BTWN BUDDE (OM) AND VINNE (LMM), WHEN BUR TWR CALLED 'ALT ALERT, PULL UP 400' IMMEDIATELY.' THIS INSTRUCTION WAS FOLLOWED EVEN THOUGH ILS G/S DISPLAYED FLY DOWN. AIRPLANE ALT WAS APPROX 2300'. TWR WAS SO ADVISED. 1BUR109.5 WAS SELECTED ON #2 NAV RECEIVER AND G/S DISPLAY AGREED WITH #1. AFTER PULL UP, BOTH G/S DISPLAYS SHOWED MAX FLY DOWN SIGNAL, AND WHEN RWY WAS SIGHTED, IT WAS APPARENT THAT AIRPLANE WAS TOO HIGH AS LMM WAS APCHED, FLAPS WERE EXTENDED FROM APCH TO LNDG AND PROPS WINDMILLED TO INCREASED RATE OF DSCNT. A NORMAL LNDG WAS MADE, SLIGHTLY LONG. DURING A FLT THE PREVIOUS DAY AND ON AN ILS APCH FOLLOWING THE RPTED APCH, THE ILS G/S DISPLAY INDICATED NORMALLY. THERE WAS NO SAFETY OF FLT INVOLVED IN THE RPTED OCCURRENCE, BUT UNDER OTHER CONDITIONS, THERE COULD BE. THIS OCCURRENCE WAS DISCUSSED WITH AN EXPERIENCED AND KNOWLEDGEABLE FAA EMPLOYEE AND THE FOLLOWING POSSIBLE CONDITIONS FOR THIS OCCURRENCE ARE SUGGESTED: 1) ELECTRONIC RADIO FREQ INTERFERENCE RADIATED FROM THE GND. 2) THE ALT ALERT SIGNAL TO THE CTLR MAY, UNDER CERTAIN CONDITIONS, BE GENERATED BY A COMPUTED VALUE, BASED ON RATE OF CHANGE OF ALT, SPD (DISTANCE), RATHER THAN AN ACTUAL INSTANTANEOUS AIRPLANE ALT.

Data retrieved from NASA's ASRS site as of August 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.