There are several variations of "low level altitude capture" and probably the most common is when ALT CAP occurs after takeoff before CLB Thrust is selected. This is usually due to a very rapid climb rate at "low-ish" take-off masses and can occur as early as 800 ft agl even with up to 5000 ft set in the MCP altitude window. The following is a look at this and other possible scares.
During the initial climb phase using either FLCH or VNAV SPD, the elevators control airspeed whilst the autothrottle is in THR HLD and so maintains the thrust achieved on take-off. When CLB thrust is selected the AutoThrottle mode changes to EPR and power adjusts to the required climb thrust. When ALT CAP is annunciated the elevators switch to controlling the altitude or the trajectory used in capturing that altitude (more of this later) and the AutoThrottle switches to SPD and now controls airspeed according to the command airspeed bug.
If, however ALT CAP is annunciated before CLB thrust is selected, the AutoThrottle remains in THR HLD holding T/O thrust and the elevators now control altitude; there is NOTHING therefore, controlling airspeed and so if left unattended the flap limit speeds can rapidly be exceeded.
Required action is to either:
Although option a) is preferred, it does require prompt recognition and reaction. Remember that there can be no AutoThrottle mode change unless CLB Thrust is first selected.
If ALT CAP occurs after CLB thrust is selected, but before acceleration to Vref 80 or higher is completed, then the elevators control altitude whilst the AutoThrottle controls speed. BUT it will control to the indicated airspeed existing at the instant of ALT CAP. This can cause the command airspeed bug to return to-a low speed if acceleration has only just begun or not yet started.
Required action: Select command speed bug to Vref 80 or, if desired, 210 kts (limit speed for Flap 15) to commence acceleration and flap retraction. Ideally this should be done as soon as ALT CAP annunciates and so avoid thrust reduction.
Another more subtle problem can present itself when ALT CAP occurs at an altitude below that which is set in the Climb page of the FMC and is properly recognised and dealt with so that acceleration and flap retraction is completed. If VNAV SPD is then used, after being cleared to a higher altitude, the airspeed bug can slew to V2 +15 as entered in the climb page as this constraint has not yet been passed. It is therefore advisable to either delete this speed constraint before further climb or use FLCH until well past that constraint. VNAV Speed intervention is another option. The foregoing assumes that no pre-flight action was taken to avoid this type of situation.
Early recognition of such problems can be helped by checking the AutoThrottle mode every time ALT CAP is annunciated and the appropriate action taken as described. There is, however, another and potentially dangerous situation which can occur if there is a sudden energy loss during the ALT CAP phase.
When the aircraft is approaching the MCP target altitude the AFDS calculates a trajectory with which to capture that altitude based upon instantaneous altitude difference (i.e. present versus target) and closure rate. At the moment of ALT CAP this trajectory is memorised and the aircraft will continue or ATTEMPT to continue on that path come what may, ie it will sacrifice airspeed, if necessary, in order to maintain that trajectory, It is important to understand that there is effectively no AFDS speed protection or control during the ALT CAP phase (the B757 is not alone in this, most modern aircraft behave in the same way). Any speed control, that MAY be present, can only come from the AutoThrottle SPD mode, which by definition must have sufficient energy from the engines to provide the speed control.
Provided there is sufficient energy available this is not a problem and, of course, is the norm 99.9% of the time. But if there is a sudden loss of energy caused by an engine failure or even by a pilot rapidly reducing thrust in anticipation of flap limit speeds being threatened then airspeed may be sacrificed in lieu of flight path. In this circumstance, airspeed can decay to well below safe values. If this situation is combined with asymmetric thrust then flight path control can be rapidly compromised. For those familiar with the A330 test flight at Toulouse some years ago, this was a worst case example of this scenario.
PILOT INTERVENTION IS REQUIRED. Control the yaw with rudder as normal and disconnect the A/P, applying maximum available thrust and pushing the nose down to regain airspeed. Bearing in mind that this may well be close to the ground, terrain contact may be the next problem, so a balance of speed/height trade off may be required, as in windshear recovery procedures, but the difference here is that there will be considerably less energy available with an engine failure.
Having said that there is no speed control as such in the ALT CAP phase, this is not strictly true. On later B757 aircraft and earlier modified ones, the AFDS will command a pitch down to try to maintain or achieve V2 plus a small margin whilst still trying to achieve the target altitude. However, if there is not enough energy for this then the Flight Director pitch bar will bias out of view. This is often mistaken for a Flight Director malfunction but is simply the Flight Director saying "I cannot help anymore". Additionally on the EADI an amber line will be drawn through the ALT CAP annunciation, indicating that this mode can no longer be achieved via the AFDS.
Although the term used is Low Level Alt Cap, it should be noted that this can be a problem at ANY altitude. For example if speed intervention (or FLCH) is used to expedite climb and is perhaps overdone and too low a value is selected resulting in significant vertical speeds causing ALT CAP to occur at a relatively low speed or Mach the potential for problems is still there.
Boeing Fleet Newsletter