  The sensitivity of beam dump experiments to heavy neutral leptons depends on
the relative strength of their couplings to individual lepton flavours in the
Standard Model. We study the impact of present neutrino oscillation data on
these couplings in the minimal type I seesaw model and find that it
significantly constrains the allowed heavy neutrino flavour mixing patterns. We
estimate the effect that the DUNE experiment will have on these predictions. We
then discuss implication that this has for the sensitivity of the NA62
experiment when operated in the beam dump mode and provide sensitivity
estimates for different benchmark scenarios. We find that the sensitivity can
vary by almost two orders of magnitude for general choices of the model
parameters, but depends only weakly on the flavour mixing pattern within the
parameter range that is preferred by neutrino oscillation data.
