I think that just measuring the number of feet of ice above the plane wouldn't be a very accurate way to determine whether the planes had sunk or not, which they likely did. What would also need to be measured would be the thickness of the ice below the planes at the time of landing and the time of recovery. So far, I haven't found any data on that. But, hypothetically, if the depth of the glacier were 3000 feet when the planes landed and the depth below the planes was only 2800 feet when they were recovered, then one could suspect that the planes did actually sink, but with the added accumulation of 60 feet of ice for a total depth of 3060 feet.
And since a significant amount of glacial melting occurs underneath glaciers, which can also affect the rate of of glacial travel, the entire glacier itself is likely sinking down, wearing away and melting from the bottom as more snow and ice accumulate on top. So that would have to be factored in too. Several feet of snow can also compress down to just a few inches of ice with added weight on top. So there are many variables at play here.
What would be cool would be to use several metal objects of substantial mass with GPS devices inside and place them in a vertical line at various depths in the ice, including both extremes--one on the surface of the ice sheet and one on the bedrock below--for a few seasons to see how far they move vertically and laterally. The bedrock itself also moves according the weight on top, including variations in atmospheric pressure. Those data would paint a more vivid picture of what happens to non-ice objects on top of glaciers over time.