Date: 201403
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Flight conditions: IMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Autothrottle/Speed Control
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Situational Awareness; Training / Qualification
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Procedure; Manuals; Human Factors
Primary problem: Aircraft

Narrative: We were cleared for an ILS approach.  I was in a descent on a base leg turning to final.  I had previously selected FL CH and was eventually cleared to 3;000 feet; and; when I was cleared for the approach; I armed LOC and then APP.  We were also previously given 170 knots and then told to maintain that until (approximately) 5 mile final.  When the aircraft reached the MCP selected 3;000 feet altitude; it leveled; and then; as I was looking to acquire the airport; and monitoring for localizer intercept; and correct tracking; the airplane chirped at me as we started to descend down the GS from our previously assigned altitude. I failed to notice that when we had leveled; the auto throttles did not 'wake up' and control our airspeed and our airspeed decreased well below my commanded value of 170 knots.  The First Officer called out 'Airspeed;' which I did not immediately hear; airspeed was called again; and additional flaps were selected.  I gained awareness of my airspeed deviation and manually applied power.  We proceeded down the glideslope and landed uneventfully.   After landing; we reviewed the approach; and I recalled how our event seemed eerily similar to an element of the recent Asiana SFO crash; where the auto-throttle 'not awakening' was identified as an issue.  The First Officer and I determined that because I was descending in FL CH; 'sometimes' the auto-throttle will not control airspeed after the auto-pilot levels the aircraft...  I determined to do a little more research on FL CH.    When I reviewed our AOM; via an iBooks search of 'FL CH'; I found 19 instances of its use.... I found the following Caution note; 'CAUTION: WHEN DESCENDING IN FL CH WITH THE AUTOPILOT DISENGAGED AND THE FLIGHT  DIRECTORS ON; THE AUTOTHROTTLE REVERTS TO THR HLD. THE PF MUST MANAGE THRUST MANUAL AS  LONG AS THE AUTOTHROTTLE ANNUNCIATES THR HLD. YOU CAN ENGAGE SPD MODE BY: 'Selecting SPD (autoflight pitch mode reverts to V/S). Capturing the glideslope or MCP altitude.'  This was the only FL CH Caution I found and the closest I could come to finding a passage that addressed a possible 'danger' of FL CH use.  However; it does not really address what happened to me.  When I captured MCP altitude of 3;000 feet; the Auto Throttle Speed Mode did NOT engage as stated in the Caution.  Also that Caution; seems entirely inadequate in addressing my situation; which; first off; was fully automated; not with Autopilot Disengaged; as the passage references.  Secondly; the very consequential issue of possibly leveling in FL CH in THR HLD; and the auto throttle NOT REAWAKENING and managing airspeed thus allowing the aircraft to slow below commanded airspeed; is not specifically addressed there at all.We need at least two things to avoid recurrence of this event: proper identification of this potential trap in our manuals; and better training regarding this dangerous condition.  First off; our manuals should clearly state that there is a condition wherein the Auto-throttle will not 'reawake' to manage airspeed when leveling after descending in FL CH.  This should be clearly identified; discussed; and cautioned against repeatedly in the various applicable sections of the AOM; probably as well as in the ASM.  Secondly; our training should emphasis this potentially dangerous condition; as well as demonstrate how it can occur in the simulator; to raise awareness of the condition so it can be properly monitored for.  Finally; perhaps; we might want to consider addressing our procedures and 'prohibiting' FL CH descents at some point during approach phase?  I don't know if that extreme step is necessary; but it would seem a simply precaution given the seriousness of possible repercussions of 'sometimes' unmanaged airspeed.  I know as a 'technique' I will attempt to never use FL CH during my approaches; and always change my FL CH descents to VS descents as I near the airport environment.




Synopsis: B757-200 Captain reported that the aircraft slowed 'well below' the selected value of 170 knots after descending in FLCH mode and commencing glideslope descent in APP with autothrottles armed. Reporter stated that the autothrottle did not 'wake up' to control speed as anticipated and that there was no AOM reference or caution regarding this specific scenario.