Narrative:

Some 3.5 NM from the approach end of runway 28L at pit, during an autoplt coupled ILS approach to that runway, the airplane suddenly began a series of wild pitch excursions. These were induced by the autoplt's attempting to follow a series of extreme oscillations of the GS signal (manifested on both of our receivers). The PF disengaged the autoplt, but was not able to do so before the ivsi had registered both climb and descent rates on the order of 2000 FPM in both directions. As we attempted to stabilize the airplane, the GS signal began to stabilize, and we were able to continue the approach. (Fortunately, there were no passenger aboard). When we broke out of the clouds at about 400 ft AGL, we saw that a kc-135 tanker aircraft had just been taxied across, south to north, at the very approach end of our landing runway, with its tail at this point just beginning to (approximately) clear the ILS critical area. At about this time, a falcon jet on approach immediately behind us said emphatically that they were experiencing problems with the GS, and asked if something was wrong. The controller responded that a beechjet (ours), just ahead of them, had not reported any problems. I immediately responded that we were reporting major problems 'now' and pointed out the kc-135 exiting the runway on the north side. (A second kc-135 was holding short of the runway to the immediate southeast of the threshold, and that aircraft, too, was in all likelihood penetrating the ILS critical area.) the rest of the approach and landing was uneventful, but before shutting down the radios at the parking area, I requested the telephone number for the pit tower. Immediately upon entering the FBO terminal, I called the tower supervisor and told him of our incident. He seemed basically unconcerned, but said that he would 'pass it along to the tower.' in retrospect, we probably should have executed a missed approach at the first signs of anomaly (albeit, the interference had produced 'no' navigation flags), but by the time we regained some semblance of control of the situation, we were in a position to continue to a normal landing. However, our altitude and rate of descent awareness were severely compromised during the incident, and from part 121 air carrier experience, it was evident that a GPWS would have alerted us to any imminent danger. It is my strong belief that all aircraft certified under part 25 of the FARS plus 'any' aircraft required to be equipped with a cockpit voice recorder should be required to be equipped (or retrofitted) with GPWS. Further, air traffic controllers need to be continually reminded of the vigilance necessary to keep vehicles -- particularly large ones -- clear of ILS critical areas during approach operations in IMC. Callback conversation with reporter revealed the following information: the reporter was flying a beech jet-400 and he says that they initially simply held their attitude and waited for the GS to stabilize (they were just inside the marker) once they disconnected the autoplt. The reporter also called a pilot group and talked to their airports and facilities specialist and he talked to the pit quality assurance personnel. The pit quality assurance people briefed this incident to the controllers and made sure that such sits do not recur.

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Original NASA ASRS Text

Title: ARPT PROB ILS. FLC ENCOUNTERS GREAT VARIATIONS IN THE GS DURING APCH IN IFR WX.

Narrative: SOME 3.5 NM FROM THE APCH END OF RWY 28L AT PIT, DURING AN AUTOPLT COUPLED ILS APCH TO THAT RWY, THE AIRPLANE SUDDENLY BEGAN A SERIES OF WILD PITCH EXCURSIONS. THESE WERE INDUCED BY THE AUTOPLT'S ATTEMPTING TO FOLLOW A SERIES OF EXTREME OSCILLATIONS OF THE GS SIGNAL (MANIFESTED ON BOTH OF OUR RECEIVERS). THE PF DISENGAGED THE AUTOPLT, BUT WAS NOT ABLE TO DO SO BEFORE THE IVSI HAD REGISTERED BOTH CLB AND DSCNT RATES ON THE ORDER OF 2000 FPM IN BOTH DIRECTIONS. AS WE ATTEMPTED TO STABILIZE THE AIRPLANE, THE GS SIGNAL BEGAN TO STABILIZE, AND WE WERE ABLE TO CONTINUE THE APCH. (FORTUNATELY, THERE WERE NO PAX ABOARD). WHEN WE BROKE OUT OF THE CLOUDS AT ABOUT 400 FT AGL, WE SAW THAT A KC-135 TANKER ACFT HAD JUST BEEN TAXIED ACROSS, S TO N, AT THE VERY APCH END OF OUR LNDG RWY, WITH ITS TAIL AT THIS POINT JUST BEGINNING TO (APPROX) CLR THE ILS CRITICAL AREA. AT ABOUT THIS TIME, A FALCON JET ON APCH IMMEDIATELY BEHIND US SAID EMPHATICALLY THAT THEY WERE EXPERIENCING PROBS WITH THE GS, AND ASKED IF SOMETHING WAS WRONG. THE CTLR RESPONDED THAT A BEECHJET (OURS), JUST AHEAD OF THEM, HAD NOT RPTED ANY PROBS. I IMMEDIATELY RESPONDED THAT WE WERE RPTING MAJOR PROBS 'NOW' AND POINTED OUT THE KC-135 EXITING THE RWY ON THE N SIDE. (A SECOND KC-135 WAS HOLDING SHORT OF THE RWY TO THE IMMEDIATE SE OF THE THRESHOLD, AND THAT ACFT, TOO, WAS IN ALL LIKELIHOOD PENETRATING THE ILS CRITICAL AREA.) THE REST OF THE APCH AND LNDG WAS UNEVENTFUL, BUT BEFORE SHUTTING DOWN THE RADIOS AT THE PARKING AREA, I REQUESTED THE TELEPHONE NUMBER FOR THE PIT TWR. IMMEDIATELY UPON ENTERING THE FBO TERMINAL, I CALLED THE TWR SUPVR AND TOLD HIM OF OUR INCIDENT. HE SEEMED BASICALLY UNCONCERNED, BUT SAID THAT HE WOULD 'PASS IT ALONG TO THE TWR.' IN RETROSPECT, WE PROBABLY SHOULD HAVE EXECUTED A MISSED APCH AT THE FIRST SIGNS OF ANOMALY (ALBEIT, THE INTERFERENCE HAD PRODUCED 'NO' NAV FLAGS), BUT BY THE TIME WE REGAINED SOME SEMBLANCE OF CTL OF THE SIT, WE WERE IN A POS TO CONTINUE TO A NORMAL LNDG. HOWEVER, OUR ALT AND RATE OF DSCNT AWARENESS WERE SEVERELY COMPROMISED DURING THE INCIDENT, AND FROM PART 121 ACR EXPERIENCE, IT WAS EVIDENT THAT A GPWS WOULD HAVE ALERTED US TO ANY IMMINENT DANGER. IT IS MY STRONG BELIEF THAT ALL ACFT CERTIFIED UNDER PART 25 OF THE FARS PLUS 'ANY' ACFT REQUIRED TO BE EQUIPPED WITH A COCKPIT VOICE RECORDER SHOULD BE REQUIRED TO BE EQUIPPED (OR RETROFITTED) WITH GPWS. FURTHER, AIR TFC CTLRS NEED TO BE CONTINUALLY REMINDED OF THE VIGILANCE NECESSARY TO KEEP VEHICLES -- PARTICULARLY LARGE ONES -- CLR OF ILS CRITICAL AREAS DURING APCH OPS IN IMC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A BEECH JET-400 AND HE SAYS THAT THEY INITIALLY SIMPLY HELD THEIR ATTITUDE AND WAITED FOR THE GS TO STABILIZE (THEY WERE JUST INSIDE THE MARKER) ONCE THEY DISCONNECTED THE AUTOPLT. THE RPTR ALSO CALLED A PLT GROUP AND TALKED TO THEIR ARPTS AND FACILITIES SPECIALIST AND HE TALKED TO THE PIT QUALITY ASSURANCE PERSONNEL. THE PIT QUALITY ASSURANCE PEOPLE BRIEFED THIS INCIDENT TO THE CTLRS AND MADE SURE THAT SUCH SITS DO NOT RECUR.

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.