LOSS OF VNAV MODE AFTER TKOF IN A B757 RESULTS IN A SPD DETERIORATION TO VERY NEAR STICK SHAKER WHEN FAILURE IS NOT RECOGNIZED IN A TIMELY MANNER.

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AS DISCUSSED BELOW; IT WILL BE SEEN THAT FMS-VNAV PROCS ARE VULNERABLE TO CERTAIN MODE CTL PANEL FAILURES AND ARE NOT FAIL-SAFE; BUT RATHER CONTRIBUTE TO A MUCH HIGHER RISK TO AVIATION SAFETY. THE MALFUNCTION HAS A VNAV MODE FAILURE; WHICH WHEN SELECTED; FAILED TO PROPERLY ENGAGE. IN OUR SIT VNAV INITIALLY ENGAGED PROPERLY; BUT ONLY REMAINED ENGAGED FOR A BRIEF PERIOD(5-10 SECONDS). HOWEVER; DURING THAT TIME VNAV DECEPTIVELY APPEARED TO OPERATE NORMALLY; INCLUDING APPROPRIATE FLT DIRECTOR COMMANDS; SALMON AIRSPD BUG MOVEMENT AND THROTTLE REDUCTION. WITH THIS; THE PF WAS INSIDIOUSLY LURED INTO BELIEVING THE SYS WAS OPERATING NORMALLY. MOMENTS LATER; VNAV TRIPPED OFF AND REVERTED TO THE PREVIOUSLY SELECTED SPD MODE (128 KTS-V2). THIS OCCURRED IN A CRITICAL PHASE OF FLT.; IMMEDIATELY AFTER TKOF; DURING FLAP RETRACTION. IT IS IMPORTANT TO NOTE THAT THE PF MAY NOT REALIZE THE VNAV MODE DISENGAGED FOR SOME TIME DUE TO THE OPERATIONAL DEMANDS OF THIS PHASE OF FLT. PARTICULARLY SINCE FLT DIRECTOR COMMANDS APPEAR REASONABLY NORMAL. ONCE THE VNAV REVERTED TO SPD MODE; THE SALMON SPD BUG DROPPED TO 128 (V2). ACTUAL SPD WAS 230 KTS AND MINIMUM MANEUVERING (FO) WAS 224. AT THIS TIME THE THROTTLES SLOWLY REDUCED TO IDLE; AND FO COMMANDED ADDITIONAL (INCREASED) PITCH UP. THE PNF WAS PRE-OCCUPIED WITH FLAPS AND A FREQ CHANGE. THE PF WAS PRE-OCCUPIED MANEUVERING BTWN HVY WX. COMMANDED PITCH WITH IDLE THRUST RESULTED IN A RAPID SPD BLEED OFF. RECOVERY WAS INITIATED AT 170 KTS. NOTE THAT; HAD INTERVENTION NOT OCCURRED; A STALL WOULD HAVE BEEN INEVITABLE. PROCEDURALLY; THE USE OF A STALL WOULD HAVE BEEN INEVITABLE. PROCEDURALLY; THE USE OF A FLT LEVEL CHANGE AS AN ALTERNATIVE TO VNAV; AT CRITICALLY LOW ALTS; COMPLETELY ELIMINATES THE SAFETY RISKS ASSOCIATED WITH THIS MALFUNCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CB REVEALED: NO FAILURE INDICATION WAS NOTED ON THE PRIMARY FLT DISPLAY; BUT IT COULD HAVE BEEN THERE. THE PF WAS CONSIDERED EXPERIENCED; BUT DOES NOT GUARD THROTTLES WHEN THEY ARE IN THE AUTO MODE. ERRONEOUS INDICATIONS WERE DISREGARDED DURING THE RECOVERY BY 'FLYING THROUGH' (DISREGARDING) THE FLT DIRECTOR. TKOF WAS MADE WITH MAX RATHER THAN REDUCED THRUST DUE TO WX CONDITIONS. THE ACFT WAS LIGHT; SO PITCH WAS RESTR TO 20 DEGS NOSE UP PER PROC.