Consider transition in the boundary layer on a flat plate or on a wing of finite span. Instability will set in at a certain distance from the leading edge and downstream of it the flow will become fully turbulent by regular transition. If locally at the leading edge a disturbance is present, the boundary layer can become turbulent just behind this disturbance. In that case a “turbulent wedge” appears in the otherwise laminar flow regime, with the typical half angle of approximately 7°, which downstream merges with the turbulent flow, Fig. 8.9 a). Only a small part of the laminar flow regime is affected.
At the leading edge of a swept wing the situation can be very different, Fig. 8.9 b). A turbulent wedge can spread out in span-wise direction, “contaminating” the originally laminar flow regime between the disturbance location and the wing tip. On a real aircraft with swept wings it is the turbulent boundary layer of the fuselage which contaminates the otherwise laminar flow at the leading edge .
The low-speed flow criterion 
illustrates well the physical background. Here sin ^u^ is the component of the external inviscid flow along the leading edge in the span-wise (wing-tip) direction, due/dx*)le. the gradient of the external inviscid flow in direction normal to the leading edge at the leading edge, and v the kinematic viscosity. Experimental data show that Ree ^ 100 ± 20 is the critical value, and that for Ree ^ 240 “leading-edge contamination”, as it was termed originally, fully happens. (For a more detailed discussion see .)
We see from that criterion the following: the larger the external inviscid flow component in the span-wise (wing-tip) direction, and the smaller the acceleration of the flow normal to the leading edge, the larger the tendency of leading – edge, or more in general, attachment-line contamination. Otherwise only a turbulent wedge would show up from the location of the disturbance in the chord – wise direction, similar to that shown in Fig. 8.9 a), however skewed.
“Contamination” can happen on general attachment lines, for instance, on those at the lower side of a flat blunt-nosed delta wing or fuselage configuration, Fig. 8.9 c). If, for instance, the TPS of a RV has a misaligned tile lying on the attachment line, turbulence can be spread prematurely over a large portion of the lower side of the flight vehicle. This argument was brought forward by D. I.A. Poll  in order to explain transition phenomena observed on the Space Shuttle Orbiter during re-entry, see also the discussion in .
The effect of attachment-line contamination in this case would be— temporally, until further down on the trajectory the ordinary transition
occurs—large and asymmetric thermal loads, a drag increase (which is not a principle problem for a RV), but also a yaw moment, whose magnitude depends on size and location of the contaminated surface part.
Attachment-line contamination in high-speed flows was studied since the 1960s, see the overviews in  and . Poll made an extensive study of attachment-line contamination at swept leading edges for both incompressible and compressible flows in the 1970s . Today, still all prediction capabilities concerning attachment-line contamination rely on empirical data. See in this regard also .