The media have been rightly confused by the story which says that Singapore radar saw MH370 descend to 5,000ft from 35,000ft after switching off the transponder. The proposed reason for the descent is that at such a low altitude even the military radar couldn’t see them.
So why could Singapore radar see them, then?
5,000ft is not a height at which any crew flies to avoid primary radar. To fly “under the radar” like military strike aircraft do in hostile territory, you fly at 250ft or lower and, if terrain allows it, you fly “nap-of-the-earth”. Your aircraft should be kicking up the dust – literally.
5,000ft is nearly 1.5km high. It is not dangerous unless you are surrounded by mountain peaks that are more than 5,000ft. But it is not a height anybody chooses to fly at if they are in a jet because fuel consumption per distance travelled at that height is so high that your range is more or less halved, compared with flying at 30,000ft or more.
So in a jet there is absolutely no good reason to be there because it confers no advantage whatsoever, and a whole lot of disadvantages.
This information only adds to the truth that whoever was doing this, if indeed it was deliberate – and that is still only a guess – may have been a victim of the adage that a little knowledge is a dangerous thing.