Narrative:

We were cleared to cross har VORTAC FL180. Copilot set up VNAV descent to make the crossing. We were on profile in glass smooth air when we unexpectedly hit some moderate turbulence near FL190. I told copilot we better slow down (I was concerned with possible overspd and slowed to turbulence penetration speed as well as the flight attendants and passenger were still up). I knew that this would destroy the descent profile and I notified ATC that we had slowed for turbulence and now could not make crossing. He replied 'ok.' since 'ok' doest not sound like an amended clearance I am filing this report as we crossed har several hundred ft high. I don't believe there were any separation issues involved. What could I have done different? I suppose in retrospect I could have requested relief from ATC prior to leaving the descent profile, but due to the need to slow for the aforementioned reasons, I think slowing immediately was the right thing to do. This is another reason I seldom use VNAV to make an ATC crossing. I prefer to mentally compute a TOD (top of descent) that is more conservative than VNAV, which allows for errors and contingencies. However, I allow the first officer to use VNAV as the company prefers and encourages its use for fuel savings, but as I tell them it's not the company's certificate on the line and I would rather we make our crossing a mi or two early!

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Original NASA ASRS Text

Title: B737-300 FLT CREW SLOW TO NEGATE POSSIBLE TURB ENCOUNTER, ADVISE ZNY UNABLE TO MAKE XING RESTR AND RECEIVE AN 'OK' WITHOUT FURTHER COMMENT.

Narrative: WE WERE CLRED TO CROSS HAR VORTAC FL180. COPLT SET UP VNAV DSCNT TO MAKE THE XING. WE WERE ON PROFILE IN GLASS SMOOTH AIR WHEN WE UNEXPECTEDLY HIT SOME MODERATE TURB NEAR FL190. I TOLD COPLT WE BETTER SLOW DOWN (I WAS CONCERNED WITH POSSIBLE OVERSPD AND SLOWED TO TURB PENETRATION SPD AS WELL AS THE FLT ATTENDANTS AND PAX WERE STILL UP). I KNEW THAT THIS WOULD DESTROY THE DSCNT PROFILE AND I NOTIFIED ATC THAT WE HAD SLOWED FOR TURB AND NOW COULD NOT MAKE XING. HE REPLIED 'OK.' SINCE 'OK' DOEST NOT SOUND LIKE AN AMENDED CLRNC I AM FILING THIS RPT AS WE CROSSED HAR SEVERAL HUNDRED FT HIGH. I DON'T BELIEVE THERE WERE ANY SEPARATION ISSUES INVOLVED. WHAT COULD I HAVE DONE DIFFERENT? I SUPPOSE IN RETROSPECT I COULD HAVE REQUESTED RELIEF FROM ATC PRIOR TO LEAVING THE DSCNT PROFILE, BUT DUE TO THE NEED TO SLOW FOR THE AFOREMENTIONED REASONS, I THINK SLOWING IMMEDIATELY WAS THE RIGHT THING TO DO. THIS IS ANOTHER REASON I SELDOM USE VNAV TO MAKE AN ATC XING. I PREFER TO MENTALLY COMPUTE A TOD (TOP OF DSCNT) THAT IS MORE CONSERVATIVE THAN VNAV, WHICH ALLOWS FOR ERRORS AND CONTINGENCIES. HOWEVER, I ALLOW THE FO TO USE VNAV AS THE COMPANY PREFERS AND ENCOURAGES ITS USE FOR FUEL SAVINGS, BUT AS I TELL THEM IT'S NOT THE COMPANY'S CERTIFICATE ON THE LINE AND I WOULD RATHER WE MAKE OUR XING A MI OR TWO EARLY!

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.