B737 first officer reported the difficulty the aircraft has meeting the gates on the alwys one arriv...

Narrative:

It appears to me that there continues to be an issue with how the 737 VNAV is flying the sfo ALWYS1 RNAV arrival. Last week; I witnessed the airplane in VNAV path unable to comply with published speed as it descended via the arrival between hefly and arrtu. It had difficulty because the airplane inexplicably chose to cross hefly at the FL190 high end of the published altitude and thus had virtually no chance in crossing arrtu at 10000 and 250 kts.today; before flying the same ALWYS1 arrival; I took note of a message on the flight plan paperwork which recommended entering a hard altitude of 14000 at hefly. I did that. What the airplane then did in VNAV path; after receiving a descend via clearance from FL270; was to cross laane at the high end FL260 and 280 kts. Subsequently; we were left with 23.5 miles for the airplane to get from FL260 at laane to 14000 at hefly with a requirement to slow from 280 to 250 kts. Again; the airplane was unable to handle the unrealistic altitude and speed parameters. In hindsight; I could have entered another hard altitude of FL220 at laane to create a more flyable path; but I don't understand why the VNAV logic wouldn't have done that planning on its own.

Original NASA ASRS Text

Title: B737 First Officer reported the difficulty the aircraft has meeting the gates on the ALWYS ONE ARRIVAL (RNAV) route into SFO even after entering some of the altitudes as hard altitudes in the VNAV PATH mode.

Narrative: It appears to me that there continues to be an issue with how the 737 VNAV is flying the SFO ALWYS1 RNAV Arrival. Last week; I witnessed the airplane in VNAV PATH unable to comply with published speed as it descended via the arrival between HEFLY and ARRTU. It had difficulty because the airplane inexplicably chose to cross HEFLY at the FL190 high end of the published altitude and thus had virtually no chance in crossing ARRTU at 10000 and 250 kts.Today; before flying the same ALWYS1 Arrival; I took note of a message on the flight plan paperwork which recommended entering a hard altitude of 14000 at HEFLY. I did that. What the airplane then did in VNAV PATH; after receiving a descend via clearance from FL270; was to cross LAANE at the high end FL260 and 280 kts. Subsequently; we were left with 23.5 miles for the airplane to get from FL260 at LAANE to 14000 at HEFLY with a requirement to slow from 280 to 250 kts. Again; the airplane was unable to handle the unrealistic altitude and speed parameters. In hindsight; I could have entered another hard altitude of FL220 at LAANE to create a more flyable path; but I don't understand why the VNAV logic wouldn't have done that planning on its own.

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.

Attributes
ACN	1339146
Time
Date	201603
Local Time Of Day	0001-0600
Place
Locale Reference	ZOA.ARTCC
State Reference	CA
Environment
Light	Night
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR ALWYS1
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Undershoot Deviation - Procedural Published Material / Policy Deviation - Speed All Types

B737 first officer reported the difficulty the aircraft has meeting the gates on the alwys one arrival (RNAV) route into sfo even after entering some of the altitudes as hard altitudes in the VNAV path mode.