A B737-800 first officer temporarily loses control of the aircraft during a wild gyration of the GS ...

Attributes
ACN	508388
Time
Date	200104
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : iah.bcstn
State Reference	TX
Altitude	msl single value : 2000
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Controlling Facilities	tracon : i90.tracon
Operator	common carrier : air carrier
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 27 other
Route In Use	approach : instrument precision approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	controller : approach flight crew : first officer
Qualification	pilot : multi engine pilot : cfi pilot : atp
Experience	flight time last 90 days : 210 flight time total : 9500 flight time type : 1500
ASRS Report	508388
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : published procedure other anomaly
Independent Detector	atc equipment : msaw other flight crewa other other : ctlr 3
Resolutory Action	controller : issued advisory flight crew : returned to assigned altitude flight crew : regained aircraft control
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance Airspace Structure Navigational Facility
Primary Problem	Navigational Facility

Narrative:

During the approach into iah on a uio-iah flight in a B737-800 aircraft, the aircraft initiated a dive. I was flying with the autoplt on. Captain was PNF, an international relief officer was observing. WX was VFR with the airport in sight. While tracking the localizer to runway 27 inbound, we were intercepting the GS from below, level at 2000 ft. I noticed the GS indication quickly (within 1-2 seconds) move from the top to the bottom on the ADI. The autoplt immediately initiated an aggressive descent to capture the false indication. I turned off the autoplt and smoothly turned the descent into a climb back to 2000 ft. The bottom of the descent was 1600 ft. At that point, I manually flew the localizer and GS indications as the captain attempted various modes on the flight director. At 1200 ft on the south, the tower issued a low altitude alert. We said we showed right on the GS. He said he didn't think 1200 was correct for our position and asked if we had the field in sight. We said yes. He cleared us for a visual. At this point, I was losing all faith in our guidance. I rationalized the situation by deciding that we must both have the parallel runway tuned so I changed frequencys. The captain said that was not the case (which it wasn't) and I asked him to retune me back to the correct frequency. The captain also stated and I agreed that the sight picture looked correct. We made a normal landing and taxied to the gate. At the gate, I asked the international relief officer if he saw anything I might have done to cause the situation and he assured me I had done nothing abnormal -- it was just one of those electronic glitches. We were following a B757 and I have heard of signals being disrupted by other aircraft. As I look back, we could have done a go around, but we were visual the entire way and the guidance wasn't required. We could have confessed our earlier altitude deviation caused by the autoplt to the controller, but we didn't and we were already busy. If runway 27 at iah had a VASI or PAPI, much of the momentary confusion could have been avoided. The controller probably had the altitude alert based on the earlier deviation and just delayed the call till we were actually right on the GS at 1200 ft. Looking back, we should have considered making a logbook write-up, although I don't think the problem could have been reproduced by maintenance.

Original NASA ASRS Text

Title: A B737-800 FO TEMPORARILY LOSES CTL OF THE ACFT DURING A WILD GYRATION OF THE GS FROM FULL UP TO FULL DOWN ON A COUPLED APCH INTO IAH, TX.

Narrative: DURING THE APCH INTO IAH ON A UIO-IAH FLT IN A B737-800 ACFT, THE ACFT INITIATED A DIVE. I WAS FLYING WITH THE AUTOPLT ON. CAPT WAS PNF, AN IRO WAS OBSERVING. WX WAS VFR WITH THE ARPT IN SIGHT. WHILE TRACKING THE LOC TO RWY 27 INBOUND, WE WERE INTERCEPTING THE GS FROM BELOW, LEVEL AT 2000 FT. I NOTICED THE GS INDICATION QUICKLY (WITHIN 1-2 SECONDS) MOVE FROM THE TOP TO THE BOTTOM ON THE ADI. THE AUTOPLT IMMEDIATELY INITIATED AN AGGRESSIVE DSCNT TO CAPTURE THE FALSE INDICATION. I TURNED OFF THE AUTOPLT AND SMOOTHLY TURNED THE DSCNT INTO A CLB BACK TO 2000 FT. THE BOTTOM OF THE DSCNT WAS 1600 FT. AT THAT POINT, I MANUALLY FLEW THE LOC AND GS INDICATIONS AS THE CAPT ATTEMPTED VARIOUS MODES ON THE FLT DIRECTOR. AT 1200 FT ON THE S, THE TWR ISSUED A LOW ALT ALERT. WE SAID WE SHOWED R ON THE GS. HE SAID HE DIDN'T THINK 1200 WAS CORRECT FOR OUR POS AND ASKED IF WE HAD THE FIELD IN SIGHT. WE SAID YES. HE CLRED US FOR A VISUAL. AT THIS POINT, I WAS LOSING ALL FAITH IN OUR GUIDANCE. I RATIONALIZED THE SIT BY DECIDING THAT WE MUST BOTH HAVE THE PARALLEL RWY TUNED SO I CHANGED FREQS. THE CAPT SAID THAT WAS NOT THE CASE (WHICH IT WASN'T) AND I ASKED HIM TO RETUNE ME BACK TO THE CORRECT FREQ. THE CAPT ALSO STATED AND I AGREED THAT THE SIGHT PICTURE LOOKED CORRECT. WE MADE A NORMAL LNDG AND TAXIED TO THE GATE. AT THE GATE, I ASKED THE IRO IF HE SAW ANYTHING I MIGHT HAVE DONE TO CAUSE THE SIT AND HE ASSURED ME I HAD DONE NOTHING ABNORMAL -- IT WAS JUST ONE OF THOSE ELECTRONIC GLITCHES. WE WERE FOLLOWING A B757 AND I HAVE HEARD OF SIGNALS BEING DISRUPTED BY OTHER ACFT. AS I LOOK BACK, WE COULD HAVE DONE A GAR, BUT WE WERE VISUAL THE ENTIRE WAY AND THE GUIDANCE WASN'T REQUIRED. WE COULD HAVE CONFESSED OUR EARLIER ALTDEV CAUSED BY THE AUTOPLT TO THE CTLR, BUT WE DIDN'T AND WE WERE ALREADY BUSY. IF RWY 27 AT IAH HAD A VASI OR PAPI, MUCH OF THE MOMENTARY CONFUSION COULD HAVE BEEN AVOIDED. THE CTLR PROBABLY HAD THE ALT ALERT BASED ON THE EARLIER DEV AND JUST DELAYED THE CALL TILL WE WERE ACTUALLY R ON THE GS AT 1200 FT. LOOKING BACK, WE SHOULD HAVE CONSIDERED MAKING A LOGBOOK WRITE-UP, ALTHOUGH I DON'T THINK THE PROB COULD HAVE BEEN REPRODUCED BY MAINT.

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.

A B737-800 first officer temporarily loses control of the aircraft during a wild gyration of the GS from full up to full down on a coupled approach into iah, tx.