M2 Circling Approaches & the "Green Donut"
Some recent business jet accidents have occurred during circling approaches in low visibility. I want to take a look at some of the factors associated with circling approaches, and how those factors could contribute to stalling an aircraft at an altitude that is difficult, if not impossible to recover from.
Key components to consider:
Circling approaches; potentially multiple turns required to maneuver to the runway for landing
Competency/recency for pilots conducting circling approaches
Increased stall speed from higher bank angle and increased wing loading
Garmin’s Approach Speed Cue (Green Donut) which is displayed on the G3000 PFD airspeed tape. The Green Donut represents 1.3 x stall speed and takes into account weight, configuration, loading
FlightSafety/Textron’s training manual for the Citation M2 recommends conducting circling approaches with landing gear down, and flaps in the approach detent at Vapproach (Vapp) +10 knots, which results in 120 knot circle speed for our video flight.
As pilots, we know that stall speed rises correlatively with the increase of bank angle and wing loading. Garmin’s Green Donut is a dynamic reference for stall speed + 30%. In the video, as we slow the aircraft, the Green Donut displays on the airspeed tape. As we make a standard rate turn (shallow bank), we can see the Green Donut move to a higher airspeed due to the resultant increased stall speed. As we tighten the turn, simulating a low visibility runway overshoot in which the pilot attempts to decrease the radius of the turn to make the landing, we can see the rapid and dramatic rise in stall speed as the Green Donut moves quickly to a higher airspeed.
What is Vapp?
FMS calculating Vapp 110 knots
Vapp is defined as 1.3 x the stall speed with gear down, and flaps in approach setting. In the video, Vapp (is calculated on the FMS at 110 KIAS, and labeled on the airspeed tape). Remember, the Green Donut takes into account the aircraft's configuration (in this case flaps approach setting), as well as bank and wing loading to give the pilot a dynamic reference to Vapp (dynamic based on angle of bank and wing loading).
The video shows that even with the 10 knot margin for circling approach (on top of the already 30% margin Vapp gives us), a pilot could quickly burn through all that margin with increased bank and wing loading to be at stall speed in a circling approach––which could result in a low altitude, unrecoverable stall. This seems to be exactly what happened at KSEE, and KTRK within the last 18 months.
I don't think it would be wise to hastily attribute what happened in these incidents to pilot incompetency. Assuming pilot error was indeed the cause of the crashes, both cases involved professional pilots who faced reasonable complications in visibility––in one case due to the time of day and the other involving smoke. It's not difficult to imagine why, in low visibility and at low altitude, even an experienced crew could be so preoccupied with maintaining visual contact with the runway environment that they might neglect critical airspeed/configuration awareness. As was demonstrated in our flight, that airspeed margin could diminish rapidly, leaving little to no time for recovery.
I recommend, especially to owner pilots (who are more likely to be flying as single pilots), to make special considerations for, or even removing these additional risk factors by refraining from circling approaches altogether.