As stated earlier the key to "capture" mode is that it is a speed controlled by thrust mode. If for some reason the thrust become limited the APFDS will sacrifice speed until the low speed protection kicks in.
Engine failure during the capture mode can be very interesting and specific handling notes may be required.
For example:
"A Flight Director (FD) guidance issue has been identified when the aircraft enters into ALTS CAP at a low selected altitude, during a high rate of climb, and at the same time an engine failure occurs. With one engine failed, the ALTS CAP guidance continues to be based on the rate of climb corresponding to both engines operating, resulting in guidance that could leads to a loss of speed below V2. There is no low speed protection based on pitch in ALTS CAP.
Velocity Decay during ALTS CAP occurs in OEI (One Engine Inoperative) conditions, either during Take-Off (TO) or Go-Around (GA).
ALTS CAP mode of operation:
Altitude on pitch
Speed on thrust (in OEI, thrust command is saturated)
During ALTS CAP, FD is flying Pitch/G command to level off at target altitude (i.e. set through Pre-Selected Altitude) and speed control is on thrust command.
This issue can occur with:
Autothrottle engaged---the Autothrottle (A/T) under speed (USPD) sub-mode gets displayed on the A/T Flight Mode Annunciator (FMA) in spite of not being effective due to lack of thrust command authority in this specific case
Autothrottle not engaged ----when the A/T USPD is not displayed on the FMA"