Airworthiness Advisory Circulars - Part 6-48 - General advice
Part 6 - General advice
Vortex Generators in Light Twin Aeroplanes
1. Several Supplemental Type Certificate (STC) vortex generator installations are available for light twin engine aeroplanes. These installations offer the advantages of reduced stall speeds, reduced Vmca, improved take-off and landing performance and increased maximum take-off weight. The actual gains available are specified in Flight Manual Supplements, and vary between aircraft types and the extent of certification testing performed by individual STC holders.
2. A flight test report of a modified aircraft advised that the claimed reduced stall speeds were not being achieved and, more importantly, stall handling qualities were degraded below acceptable limits. The unacceptable stall handling behaviour was uncontrollable wing drop at the stall.
3. Following this report an investigation was made to determine whether this type of modification should continue to be approved. The investigation involved flight assessments of other similarly modified aircraft, and consultation with the STC holder and the FAA. The investigation revealed there was not a problem with any particular STC, and that the basic concept was safe.
4. However, the maintenance of safe flying qualities is critically dependent on the the condition of the wing surface. This is because the basic mechanism by which the vortex generators affect the aeroplane is to extend the lift curve slope so that the stall occurs at a higher angle of attack. They produce almost no effect at angles of attack below the stall. Therefore, if an aircraft has some asymmetries that show up at angles of attack above the normal stall angle, they could become apparent with vortex generators installed, but not without them.
5. Such asymmetries could be a de-ice boot with a leak on the aft part of the lower surface that could cause one boot to partially inflate at high angles of attack. Other causes might be asymmetry of the external wing contours.
6. If your aircraft is, or is about to be, fitted with vortex generators, you are advised to have the external surfaces and, if installed, de-ice boots, carefully inspected for damage or asymmetry. You are further advised of the added importance of ensuring any damage is carefully, and accurately, repaired.