Motorists driving along Old Warrington Road had a lucky escape when the grill dropped out of the sky at about 8.20am last Wednesday, August 5.
It landed on an old rubbish tip just metres from the road.
Police took the grill away within minutes, and sent it back to Manchester Airport for identification.
It was traced to a Tristar jet which had taken off from Manchester Airport at 8am, bound for Lanzarote.
On Thursday, Sussex-based Air Atlanta Icelandic, which owns the jet, began an internal investigation as to why the 2ft by 2ft aluminium grill fell off.
Air Atlanta's, David Hurst, said: "The grill covers an air vent in the rear fuselage and is not a pressurised or structured part.
"The aircraft manual lists the grill as one of the parts that need not be fitted unless required for cosmetic reasons."
He added that Air Atlanta's engineers have now been ordered to check similar grills on all the company's planes.
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