The standard rule of thumb for convection ovens is to do exactly what you would do with a conventional oven, but reduce the temperature by 25 degrees. I've never roasted a 26 - 28 pound turkey, so I cannot say with any authority whether any other rule should apply. I do know that I've never had any problem with any recipe, of any kind, following the rule stated above, in the 6 years that I've had a convection oven (and which I use at least 5 times a week, for all kinds of roasting and baking). ;o)
P.S. Are you planning to put stuffing inside the bird? If so, that will affect the roasting time.
shoot for 375°F for about 3 1/2 hours, or until the thighs have an internal temp of about 165°F. this article might be helpful! http://www.sfgate.com/recipes/article/Talking-convection-oven-turkey-2548466.php
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P.S. Are you planning to put stuffing inside the bird? If so, that will affect the roasting time.