Altitude deviation. Overshoot on descent.

Attributes
ACN	122020
Time
Date	198909
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : oak
State Reference	CA
Altitude	msl bound lower : 9200 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : oak artcc : zau
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	arrival : profile descent arrival other arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 185 flight time total : 5000 flight time type : 1500
ASRS Report	122020
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : published procedure
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was pilot flying the profile descent into sfo from the south. Autoplt on, FMC properly programmed and flying in LNAV (lateral navigation) and VNAV (vertical navigation). Sfo approach control instructed us to slow from 290 KTS to 210 KTS. I disconnected the vertical navigation (VNAV) and used flight level change mode to slow the aircraft down as this seems to be a more efficient means to slow the aircraft. By doing this the FMC no longer gives intermediate level off protection but goes right to the altitude set in the altitude window. I forgot which mode I was in and expected the aircraft to level at 10,000' prior to descending to 8000' (which was set in altitude window). At 9200' I realized we still had 1 mi to go before we were cleared out of 10,000'. I leveled off momentarily then continued with the profile descent. Approach control did not mention anything about the bust nor did we. How do I prevent this in the future? Two possibilities: 1) step down with altitude window to profile altitudes rather than setting final altitude and depending on computer (FMC). This way if I change modes on the autoplt I will be protected. 2) commit to not changing modes when on the profile descent.

Original NASA ASRS Text

Title: ALT DEVIATION. OVERSHOOT ON DESCENT.

Narrative: I WAS PLT FLYING THE PROFILE DSCNT INTO SFO FROM THE S. AUTOPLT ON, FMC PROPERLY PROGRAMMED AND FLYING IN LNAV (LATERAL NAVIGATION) AND VNAV (VERTICAL NAVIGATION). SFO APCH CTL INSTRUCTED US TO SLOW FROM 290 KTS TO 210 KTS. I DISCONNECTED THE VERTICAL NAV (VNAV) AND USED FLT LEVEL CHANGE MODE TO SLOW THE ACFT DOWN AS THIS SEEMS TO BE A MORE EFFICIENT MEANS TO SLOW THE ACFT. BY DOING THIS THE FMC NO LONGER GIVES INTERMEDIATE LEVEL OFF PROTECTION BUT GOES RIGHT TO THE ALT SET IN THE ALT WINDOW. I FORGOT WHICH MODE I WAS IN AND EXPECTED THE ACFT TO LEVEL AT 10,000' PRIOR TO DESCENDING TO 8000' (WHICH WAS SET IN ALT WINDOW). AT 9200' I REALIZED WE STILL HAD 1 MI TO GO BEFORE WE WERE CLRED OUT OF 10,000'. I LEVELED OFF MOMENTARILY THEN CONTINUED WITH THE PROFILE DSCNT. APCH CTL DID NOT MENTION ANYTHING ABOUT THE BUST NOR DID WE. HOW DO I PREVENT THIS IN THE FUTURE? TWO POSSIBILITIES: 1) STEP DOWN WITH ALT WINDOW TO PROFILE ALTS RATHER THAN SETTING FINAL ALT AND DEPENDING ON COMPUTER (FMC). THIS WAY IF I CHANGE MODES ON THE AUTOPLT I WILL BE PROTECTED. 2) COMMIT TO NOT CHANGING MODES WHEN ON THE PROFILE DSCNT.

Data retrieved from NASA's ASRS site as of August 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.