Medium large transport has altitude deviation while in performance mode. TCASII RA.

Attributes
ACN	187711
Time
Date	199108
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : lit
State Reference	AR
Altitude	msl bound lower : 26800 msl bound upper : 27150
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	artcc : zme tower : mem
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 12000 flight time type : 1300
ASRS Report	187711
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were in cruise with autoplt on in performance mode. Memphis center was very busy with numerous aircraft deviating around convective activity. The aircraft was wandering +/- 150 in performance mode. With TCASII on we noticed headon traffic at FL280. We got a TA, then followed by an RA and a descend command. Our aircraft was at 27150. The first officer started the descent to satisfy the TCASII and I picked up the traffic visually and confirmed his altitude was FL280. Our descent was stopped at 26800. We then returned to FL270. As mentioned before memphis center was extremely busy and I did not advise him of our minor altitude deviation. The TCASII system worked as advertised and is a valuable aid. With the aircraft in performance mode the wandering of +/- 200 ft contributes to unnecessary and unwarranted TCASII alerts. If only altitude hold was used it would eliminate the wandering and satisfy the TCASII criteria and eliminate unwarranted TA commands.

Original NASA ASRS Text

Title: MLG HAS ALT DEV WHILE IN PERFORMANCE MODE. TCASII RA.

Narrative: WE WERE IN CRUISE WITH AUTOPLT ON IN PERFORMANCE MODE. MEMPHIS CENTER WAS VERY BUSY WITH NUMEROUS ACFT DEVIATING AROUND CONVECTIVE ACTIVITY. THE ACFT WAS WANDERING +/- 150 IN PERFORMANCE MODE. WITH TCASII ON WE NOTICED HEADON TFC AT FL280. WE GOT A TA, THEN FOLLOWED BY AN RA AND A DSND COMMAND. OUR ACFT WAS AT 27150. THE FO STARTED THE DSCNT TO SATISFY THE TCASII AND I PICKED UP THE TFC VISUALLY AND CONFIRMED HIS ALT WAS FL280. OUR DSCNT WAS STOPPED AT 26800. WE THEN RETURNED TO FL270. AS MENTIONED BEFORE MEMPHIS CENTER WAS EXTREMELY BUSY AND I DID NOT ADVISE HIM OF OUR MINOR ALT DEV. THE TCASII SYS WORKED AS ADVERTISED AND IS A VALUABLE AID. WITH THE ACFT IN PERFORMANCE MODE THE WANDERING OF +/- 200 FT CONTRIBUTES TO UNNECESSARY AND UNWARRANTED TCASII ALERTS. IF ONLY ALT HOLD WAS USED IT WOULD ELIMINATE THE WANDERING AND SATISFY THE TCASII CRITERIA AND ELIMINATE UNWARRANTED TA COMMANDS.

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.