Narrative:

On the WYNDE7 arrival into ord; the plane was cleared to descend at 300 KTS and cross wynde at 11;000 feet. After arriving at 11;000; the aircraft began slowing to 250 KTS. After looking at the descent page on the FMC; the 250 was highlighted for 10;000 or below on the descent page. This was neither commanded nor desired; as chicago center questioned us on our airspeed at the same time we realized the plane was slowing without being in the constraint parameters. A new ATIS report had arrived; and we were analyzing the crosswind and tailwind components for landing; as they were approaching limits. In the brief period we had looked at this information; the aircraft had slowed. Our aircraft had the older 737NG FMC and I was familiar with the lack of reliability and unpredictability on this model. However; I did not anticipate an aircraft slowing while level at 11;000 feet to the 10;000 feet and below speed.[our company] would like us to maximize the use of the VNAV function; but the lack of reliability and unpredictability; which is neither explained in the flight manual or trained for in the flight training; makes me question the viability of the system. Using VNAV on the older 737 ng models is like letting a 4 year old drive a car while sitting on your lap. You have to constantly watch; help and correct; but you give him all the credit. And your workload is reduced; because after all; he drove; didn't he? Its fine in an empty parking lot; but not on the freeway. VNAV has been a constant issue on this plane; and many issues probably never get documented. However; the 'what's it gonna do now?' and 'why did it do that?' factor on this plane is higher than any other aircraft I have ever flown with automation. I believe better instruction; and clarification of the realities of the system; its limits; and limitations needs to be addressed.

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Original NASA ASRS Text

Title: B737 NG Captain reported a speed deviation during arrival to ORD. Captain stated the FMS VNAV performance on older NG models is poor and requires constant monitoring.

Narrative: On the WYNDE7 Arrival into ORD; the plane was cleared to descend at 300 KTS and cross WYNDE at 11;000 feet. After arriving at 11;000; the aircraft began slowing to 250 KTS. After looking at the descent page on the FMC; the 250 was highlighted for 10;000 or below on the descent page. This was neither commanded nor desired; as Chicago Center questioned us on our airspeed at the same time we realized the plane was slowing without being in the constraint parameters. A new ATIS report had arrived; and we were analyzing the crosswind and tailwind components for landing; as they were approaching limits. In the brief period we had looked at this information; the aircraft had slowed. Our aircraft had the older 737NG FMC and I was familiar with the lack of reliability and unpredictability on this model. However; I did not anticipate an aircraft slowing while level at 11;000 feet to the 10;000 feet and below speed.[Our company] would like us to maximize the use of the VNAV function; but the lack of reliability and unpredictability; which is neither explained in the flight manual or trained for in the flight training; makes me question the viability of the system. Using VNAV on the older 737 NG models is like letting a 4 year old drive a car while sitting on your lap. You have to constantly watch; help and correct; but you give him all the credit. And your workload is reduced; because after all; he drove; didn't he? Its fine in an empty parking lot; but not on the freeway. VNAV has been a constant issue on this plane; and many issues probably never get documented. However; the 'what's it gonna do now?' and 'why did it do that?' factor on this plane is higher than any other aircraft I have ever flown with automation. I believe better instruction; and clarification of the realities of the system; its limits; and limitations needs to be addressed.

Data retrieved from NASA's ASRS site 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.