Can Transition Altitude (TA) and Transition Level (TL) be equal? In Europe, is it possible that Transition Altitude and Transition Level are equal, that is, TA=8000 ft and TL=FL080? In that scenario, at 8000 feet, what pressure (QNH or QNE) should be set in the altimeter? Is it possible that Transition Altitude is higher than Transition Level, e.g. TA=10,000 ft and TL= FL090? If the above scenarios are not possible, is there some minimum altitude of the transition layer that must always be assured?
The TA and TL will always be at least 1000ft apart to ensure that aircraft flying at each will always be properly separated.
The TA will be a fixed value to ensure terrain and obstacle clearance, while the TL will be the lowest reasonable value that provides this minimum vertical separation above the TA. However, “reasonable” includes not having any possible numerical ambiguity.
So, if your TA is 8000ft, then the lowest possible TL is FL085. Depending on the QNH, though, the TL may be higher.