AFMS N°D07 FOR GARMIN GFC700 AUTOPILOT AND VARIABLE PITCH PROPELLER EQUIPPED
AEROPLANES
Section 1 – General-
AUTOPILOT FUNCTIONS
How Underspeed Protection functions depends on which vertical flight director mode is
selected.
For the purpose of this discussion, the vertical flight director modes can be divided into two
categories:
- It is important to maintain altitude for as long as possible (altitude- crit-
ical modes).
- Maintaining altitude is less crucial (non-altitude critical modes).
If USP engages while Autopilot is in PIT or VS mode, and power is ab-
ruptly set to full throttle, pilot may expect a slight nose up tendency that
will be quickly counteracted by autopilot. This is a normal behaviour, the
slight nose up tendency in this case will be due to abrupt power advance,
but will be immediately damped by Autopilot (with PIT or VS turn-
ing green on the annunciation panel).
USP engagement is a consequence of autopilot failing to properly moni-
toring A/P and aircraft behaviour. This is mainly caused by a pilot re-
quest that cannot be fulfilled due to lack of power coupled with unattain-
able pitch input.
Pilot need to continuously monitor autopilot performance, while checking
that mode selections are compatible with aircraft performance.
If Autopilot is engaged in FLC mode, FLC mode will downgrade to PIT
mode when in USP for more than 10 seconds.
If USP engages while in ALT mode, upon USP disengagement FD will
command bars to regain pre-selected altitude (altitude selected before USP
engagement). Pilot must monitor A/P to check if previous pre-selected alti-
tude is still attainable with energy available. It is recommended in this case
to preselect a new altitude in order to speed-up energy management and
recover from slow speed conditions.
When USP becomes active, expect a small FD adjustment due to reac-
tion to very low speed condition.
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