WHAT DOES THAT YELLOW ARC ON THE AIRSPEED INDICATOR MEAN?
The RAAus Magazine for April, 2011 had an excellent article by Stewart Madigan about rough air flying. It made me think a bit about stuff I took for granted. Here is the gist of the article:
VNE – Velocity Never Exceed
VNO – Velocity Normal Operations Do not exceed except in calm air and only with caution.
VA – Design Manoeuvring speed
VSI – Velocity Stall No Flaps
The YELLOW arc is a cautionary speed range which is a function of turbulence. Hitting rising air, such as a thermal, momentarily increases the angle of attack which in turn increases the G loading on the wing and adds stress. When the G loading exceeds the design limit, say goodbye to your wings. The further the airspeed goes into the YELLOW arc, the less turbulence the wings can take.
When the air speed is near the top of the GREEN arc, the aircraft should be able to safely handle light to moderate turbulence. Common sense dictates that the more turbulence, the less airspeed, even in the GREEN arc range.
In severe turbulence the aircraft must be flown at VA, or below this speed. VA is also the maximum speed at which full control deflection can be applied.
VA is different for each aircraft. It is determined by the relationship:
VA=VSI times the square root of the design stress loading
For the Motorfalkes, VA is 36 knots times the square root of 5 g. = 81 knots. IN XJX, the yellow arc actually begins at 75 knots, so it is conservative. VNE, the top of the yellow arc, is 100 knots. Don’t go there! In fabric covered aircraft like the Motorfalke, VNE is always 100 knots (or less) if the fabric is not held to the ribs by rib stitching which keeps it from lifting at higher speeds. This could ruin your whole day as it did to Steve Wittman, a famous US aviator who is honored by Oshkosh.
For the Dimona, VA is 38 knots times the square root of 6 g. = 95 knots. In ZDD, the yellow arc begins at 110. 95 knots is a safer number to use. VNE is a whopping 145 knots.
However, don’t take any of these numbers as gospel. As aircraft age, the airframe may not still have the design g. force capabilities. Better to be safe than sorry. If the air is bumpy, slow down. If it is really bumpy, stay on the ground! (Or you may end up there anyway…..)