Ken, FWIW, even if that mast was not metallic (though I suspect it is) the coax feed(s) are, and would be distorting your pattern pretty severely to the point where you are not getting the 9-10db you should be getting out of that 6-el, in fact I'd be surprised if you were getting half, which may be why it took you so long to align it... the peaks and nulls are not where you think they should be. It also makes it a bitch to tune, and your bandwidth gets pretty narrow with six elements to begin with. Even if your mast was PVC, the coax going up to the omni would essentially make it a metal mast, so there is no escape in the present configuration.
If it were me, I would twist that beam horizontal, which even though you ordinarily would lose 3db for the vertically polarized ground antennas (over a properly mounted vertical) would make little difference with the aircraft, would reduce AM impulse interference, restore the gain and pattern, thus probably working better overall. I would also want to get it above the metallic guys, which are more problematic when they are angled closer to the plane of the elements and passing nearer to the high impedance ends than, say, terminating at a guy ring right beneath the boom of a horizontal array. If you can't get the antenna above the guys, you could at least break up the guys at their upper ends with multiple insulators, say one every 18" for any guy within four feet of an array element. Even simpler, just replace the cable with 1/4" polyester line from the marine store... stretch and sunlight resistant, absorbes shock better than cable and for your short mast will work just fine.
Otherwise, the only way to get full design performance out of a vertical yagi is to side-mount it off the mast by at least a quarter wave (2 ft), or end-mount it from the rear, though yours is getting pretty long for an end mount and "weather vaning" becomes an issue unless it is heavy-duty commercial stuff (2 and 4 el examples attached).
There again, if it's good enough as is... it's good enough. Sometimes "better" is the enemy of "good enough".