Reverse sensing of the GS causes medium large transport to descend below the GS at far.

Attributes
ACN	213313
Time
Date	199205
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : far
State Reference	ND
Altitude	agl bound lower : 1000 agl bound upper : 1000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : far
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : atp pilot : instrument
ASRS Report	213313
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	non adherence : far other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	faa : investigated
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other
Situations
Navigational Aid	Unspecified

Narrative:

On visual approach for runway 17 at far, copilot was flying and using the ILS as a backup for a visual. I noticed he was slightly low on VASI (or PAPI, whichever it is) at approximately 1000 ft AGL. He continued to go around lower and I noticed the ILS GS indicated we were high. It was wrong, we were low as correctly indicated by the VASI/PAPI. We told far ground controller. He said he would check it out. The next week he reported to us that the ILS had been getting maintenance earlier in the day. Apparently some cables were reconnected to the wrong terminals, in effect causing reverse sensing on the GS. No warning of this condition was generated. Obviously, this is dangerous and must be prevented in the future by redesigning the system.

Original NASA ASRS Text

Title: REVERSE SENSING OF THE GS CAUSES MLG TO DSND BELOW THE GS AT FAR.

Narrative: ON VISUAL APCH FOR RWY 17 AT FAR, COPLT WAS FLYING AND USING THE ILS AS A BACKUP FOR A VISUAL. I NOTICED HE WAS SLIGHTLY LOW ON VASI (OR PAPI, WHICHEVER IT IS) AT APPROX 1000 FT AGL. HE CONTINUED TO GAR LOWER AND I NOTICED THE ILS GS INDICATED WE WERE HIGH. IT WAS WRONG, WE WERE LOW AS CORRECTLY INDICATED BY THE VASI/PAPI. WE TOLD FAR GND CTLR. HE SAID HE WOULD CHK IT OUT. THE NEXT WK HE RPTED TO US THAT THE ILS HAD BEEN GETTING MAINT EARLIER IN THE DAY. APPARENTLY SOME CABLES WERE RECONNECTED TO THE WRONG TERMINALS, IN EFFECT CAUSING REVERSE SENSING ON THE GS. NO WARNING OF THIS CONDITION WAS GENERATED. OBVIOUSLY, THIS IS DANGEROUS AND MUST BE PREVENTED IN THE FUTURE BY REDESIGNING THE SYS.

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.