When at a CG forward of approximately 14" and at very low airspeeds, there may be insufficient airflow over the tailplane and elevators to maintain pitch authority. This may result in the aircraft pitching down until airspeed is regained.
It is more prevalent on the 4-place models that have been fitted with a heavier engine (such as an IO540), that therefore tend to have a more forward empty CG. It is less likely to be observed when the aircraft CG is aft of 14".
The effect is observable when on approach (and CG is forward of 14") at airspeeds below 55KIAS, and when power is reduced to idle. The resulting reduction in airflow over the elevators can cause a reduction or loss of pitch control and an accompanying high rate of descent until airspeed is regained, which may take several hundred feet of altitude.
Additional factors
In the event of an engine failure when at low airspeed and forward CG during initial takeoff and climbout, or on final approach, there may not be sufficient pitch authority to establish a glide approach, and there may not be sufficient altitude to regain airspeed.
Ensuring that the CG is in a more favorable part of the envelope can be accomplished by adding weight at an aft location. For back-country operations, a survival kit at the aft baggage area is often sufficient.
When on approach to a sufficiently long runway, there is usually no need to reduce the airspeed to below 55 KIAS until close to touchdown.
During approaches and departures to/from short airstrips when at forward CG, the exposure to airspeeds below 55KIAS should be minimized .
