Flight crew of DC9 on ILS to runway 12R at msp deviate below GS when aircraft exiting runway disrupt...

Attributes
ACN	614401
Time
Date	200403
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : msp.airport
State Reference	MN
Altitude	msl single value : 2300
Environment
Flight Conditions	IMC
Weather Elements	Fog
Light	Night
Aircraft 1
Controlling Facilities	tower : msp.tower
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 12r
Flight Phase	descent : intermediate altitude
Route In Use	approach : instrument precision
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tower : msp.tower
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	ground : position and hold
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 188 flight time total : 15440 flight time type : 6465
ASRS Report	614401
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical conflict : ground less severe other anomaly other anomaly other
Independent Detector	aircraft equipment : gpws other flight crewa
Supplementary
Problem Areas	Navigational Facility Flight Crew Human Performance
Primary Problem	Navigational Facility

Narrative:

Due to stuck microphone on tower frequency, an aircraft in position on runway elected to turn off the runway, but in doing so, turned directly in front of the GS antenna. I was later told by tower manager that they were unable to reach either our aircraft or the other aircraft because of the stuck microphone. WX at the time did not require protection of the ILS critical area. However, the GS did fluctuate significantly and after the GS beam was no longer blocked, we found ourselves significantly below the GS. Strangely, the GS deviation was subtle enough not to require large changes in pitch or power. The aircraft never departed the stabilized approach criterion required by our carrier. After the GS area was cleared, the GPWS briefly gave a mode 5 warning, but by then the PAPI was clearly visible and the aircraft returned to GS and landed without incident.

Original NASA ASRS Text

Title: FLT CREW OF DC9 ON ILS TO RWY 12R AT MSP DEVIATE BELOW GS WHEN ACFT EXITING RWY DISRUPTS GS SIGNAL.

Narrative: DUE TO STUCK MIKE ON TWR FREQ, AN ACFT IN POS ON RWY ELECTED TO TURN OFF THE RWY, BUT IN DOING SO, TURNED DIRECTLY IN FRONT OF THE GS ANTENNA. I WAS LATER TOLD BY TWR MGR THAT THEY WERE UNABLE TO REACH EITHER OUR ACFT OR THE OTHER ACFT BECAUSE OF THE STUCK MIKE. WX AT THE TIME DID NOT REQUIRE PROTECTION OF THE ILS CRITICAL AREA. HOWEVER, THE GS DID FLUCTUATE SIGNIFICANTLY AND AFTER THE GS BEAM WAS NO LONGER BLOCKED, WE FOUND OURSELVES SIGNIFICANTLY BELOW THE GS. STRANGELY, THE GS DEV WAS SUBTLE ENOUGH NOT TO REQUIRE LARGE CHANGES IN PITCH OR PWR. THE ACFT NEVER DEPARTED THE STABILIZED APCH CRITERION REQUIRED BY OUR CARRIER. AFTER THE GS AREA WAS CLRED, THE GPWS BRIEFLY GAVE A MODE 5 WARNING, BUT BY THEN THE PAPI WAS CLRLY VISIBLE AND THE ACFT RETURNED TO GS AND LANDED WITHOUT INCIDENT.

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.

Flight crew of DC9 on ILS to runway 12R at msp deviate below GS when aircraft exiting runway disrupts GS signal.