Extending the work of Koopman (1972), the Mt/Ekt/k system was proposed by Kivestu (1976) as a model that could be used to directly compute approximate queueing statistics for airports— rather than separately solving the M(t)/M(t)/k and M(t)/D(t)/k models and then somehow interpolating their results. (Note that negative exponential service times (M and constant service times (D) are simply special cases of the Erlang (Ek) family, with k = 1 and k =, respectively.) Kivestu (1976) noted that k should√k, wherebe determinedES and fromS denotethe relationshipthe expectedES /value S = and the standard deviation of the service times and can be estimated from field data. He also developed a powerful numerical approximation scheme that computes the (time varying) state probabilities for the Mt/Ekt/k system efficiently.