Large transport FMC unable to compensate for tailwinds and misses altitude crossing restriction.

Attributes
ACN	240771
Time
Date	199305
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mkg
State Reference	MI
Altitude	msl bound lower : 23000 msl bound upper : 23300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 180 flight time total : 14000 flight time type : 750
ASRS Report	240771
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : crossing restriction not met non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	none taken : insufficient time
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Automated cockpit with relatively new VNAV system, which does not do a good job with a tailwind much over 15 KTS. Crossing restriction of giber at FL230. Pilot was on other frequency, saw FL230 in altitude window, got back to first officer. He confirmed giber at FL230. We discussed VNAV 5 difficulty. He explained he had never seen it miss. I disagreed with him, explaining constraint. He selected open descent, missed giber by 300 ft. As much as you try to have good cockpit resource management and let the person make their own mistakes, you have to step in earlier. At the next crossing restriction when the same situation was occurring I intervened and directed him to start the descent earlier and in the open mode. Tailwind was 30-40 KTS. This VNAV system as I said earlier is terrible with any sort of tailwind and now you see people pushing bogus altitudes to end run the system.

Original NASA ASRS Text

Title: LGT FMC UNABLE TO COMPENSATE FOR TAILWINDS AND MISSES ALT XING RESTRICTION.

Narrative: AUTOMATED COCKPIT WITH RELATIVELY NEW VNAV SYS, WHICH DOES NOT DO A GOOD JOB WITH A TAILWIND MUCH OVER 15 KTS. XING RESTRICTION OF GIBER AT FL230. PLT WAS ON OTHER FREQ, SAW FL230 IN ALT WINDOW, GOT BACK TO FO. HE CONFIRMED GIBER AT FL230. WE DISCUSSED VNAV 5 DIFFICULTY. HE EXPLAINED HE HAD NEVER SEEN IT MISS. I DISAGREED WITH HIM, EXPLAINING CONSTRAINT. HE SELECTED OPEN DSCNT, MISSED GIBER BY 300 FT. AS MUCH AS YOU TRY TO HAVE GOOD COCKPIT RESOURCE MGMNT AND LET THE PERSON MAKE THEIR OWN MISTAKES, YOU HAVE TO STEP IN EARLIER. AT THE NEXT XING RESTRICTION WHEN THE SAME SIT WAS OCCURRING I INTERVENED AND DIRECTED HIM TO START THE DSCNT EARLIER AND IN THE OPEN MODE. TAILWIND WAS 30-40 KTS. THIS VNAV SYS AS I SAID EARLIER IS TERRIBLE WITH ANY SORT OF TAILWIND AND NOW YOU SEE PEOPLE PUSHING BOGUS ALTS TO END RUN THE SYS.

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.