<< >> NASA ASRS Report 347015, September 1996

An air carrier B737 at 5000 ft climbed 500 ft above his assigned altitude due to a TCASII RA on a PA31 below him that had inadvertently descended 100 ft from his assigned altitude of 4000 ft and then climb back. The TCASII caught the climb and the controller's ASR 9 never saw the altitude diversion of the PA31.

Attributes
ACN	347015
Time
Date	199609
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : ssf airport : sat
State Reference	TX
Altitude	msl bound lower : 5000 msl bound upper : 5500
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : sat
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	arrival star : star enroute airway : sat
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Route In Use	arrival star : star
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller military : 5 controller non radar : 15 controller radar : 19 controller supervisory : 4
ASRS Report	347015
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Miss Distance	horizontal : 0 vertical : 1000
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

Air carrier X (B737S) inbound on the STAR from the south, level at 5000 ft. Small transport Y (PA31) on same route, level at 4000 ft. According to the altitude readout on the ASR 9 both aircraft were level and nothing unusual occurred. Air carrier X transmitted and I could hear the TCASII alert in the backgnd. He asked if there was traffic. I issued traffic level at 4000 ft. I observed his altitude climbing now through 5500 ft. I asked if air carrier X was responding to TCASII on the 4000 ft traffic. Air carrier X pilot responded affirmative. The pilot of small transport Y said he had descended accidentally 100 ft and had climbed from 3900 ft to 4000 ft. The ASR 9 did not pick this up but the TCASII did. The air carrier X B737S climbed 500 ft in response to a 100 ft climb from an aircraft directly below it. ATC equipment is obviously outdated when onboard equipment is more accurate and can respond better than ATC's.

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Original NASA ASRS Text

Title: AN ACR B737 AT 5000 FT CLBED 500 FT ABOVE HIS ASSIGNED ALT DUE TO A TCASII RA ON A PA31 BELOW HIM THAT HAD INADVERTENTLY DSNDED 100 FT FROM HIS ASSIGNED ALT OF 4000 FT AND THEN CLB BACK. THE TCASII CAUGHT THE CLB AND THE CTLR'S ASR 9 NEVER SAW THE ALT DIVERSION OF THE PA31.

Narrative: ACR X (B737S) INBOUND ON THE STAR FROM THE S, LEVEL AT 5000 FT. SMT Y (PA31) ON SAME RTE, LEVEL AT 4000 FT. ACCORDING TO THE ALT READOUT ON THE ASR 9 BOTH ACFT WERE LEVEL AND NOTHING UNUSUAL OCCURRED. ACR X XMITTED AND I COULD HEAR THE TCASII ALERT IN THE BACKGND. HE ASKED IF THERE WAS TFC. I ISSUED TFC LEVEL AT 4000 FT. I OBSERVED HIS ALT CLBING NOW THROUGH 5500 FT. I ASKED IF ACR X WAS RESPONDING TO TCASII ON THE 4000 FT TFC. ACR X PLT RESPONDED AFFIRMATIVE. THE PLT OF SMT Y SAID HE HAD DSNDED ACCIDENTALLY 100 FT AND HAD CLBED FROM 3900 FT TO 4000 FT. THE ASR 9 DID NOT PICK THIS UP BUT THE TCASII DID. THE ACR X B737S CLBED 500 FT IN RESPONSE TO A 100 FT CLB FROM AN ACFT DIRECTLY BELOW IT. ATC EQUIP IS OBVIOUSLY OUTDATED WHEN ONBOARD EQUIP IS MORE ACCURATE AND CAN RESPOND BETTER THAN ATC'S.

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.