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I just finished reading an interesting post where the author suggests that the missing Malaysian Airlines flight 370 could have 'hidden' under the cover of Singapore Airlines flight 68 to fly to a covert airstrip.

I've read multiple thrillers where submarines have hidden behind the acoustic signal of large cargo vessels to mask their sound over sonar arrays, and this (at least in the book) sounds plausible. I also read another book where a bomber (flying under the identity of a commercial jet) masked a business jet to get it into an area without detection. Naturally, both examples are taken from fiction, but I'm wondering:

  • Would there be any hints now in retrospective to suggest they were being followed? Their transponder was off of course, but I'm wondering if for instance there might be some radio interference or static from such as large aircraft behind them? Are there every any issues when refueling aircraft do this sort of thing?

  • What is the resolution of radar? Would one be able to notice the two blips on the screen unless they were literally under or beside each other, or would it still appear as one dot on secondary radar if say they were 200 meters behind and below?

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4 Answers 4

up vote 19 down vote accepted

Radar resolution is defined in terms of degrees of arc (or alternatively in steradians/solid angles) and as such, the further away from the radar, the larger volume is being sampled. At altitude and some distance from the radar you should have a bit of leeway on how loose of a formation you can fly and show up as one contact on the radar display. I'm not giving credence to the theory presented, but I am saying it is plausible that one aircraft could fly a formation with another and be detected as one airplane by a radar.

If the transponder is off, the aircraft you are following will not see you on TCAS and if you are behind them you will be visually undetectable by the crew or passengers. If you fly above them you will be clear of their wake, so I would put behind and above the best position to shadow. A lateral offset might be safer, but if you stray too much, you might wander into the FOV of the cabin. I can't see any way the following airplane would cause nav or com interference in the followed airplane.

The biggest threat to being detected in this scenario are being seen by other airplane. If ATC calls traffic and you see two planes in formation instead, you would probably say something. In the case of the airplane you refer to however, it was night and if you turn all of your position lights, beacons, strobes, tail lights and if there is no moon out you might escape visual detection by a third party.

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Great answer. Surely though, manouvering into position above and behind would cause some minor heart attacks at ATC calling the lead aircraft to let them know of unknown aircraft close by. Not having a transponder doesn't mean it's undetectable. –  Paul Leigh Mar 18 at 8:50
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I believe it is practically undetectable to ATC. –  Hugh Mar 18 at 11:17
    
@PaulLeigh In that case ATC would see the primary target converging with the full target but the best ATC can say is possible traffic, altitude unknown. ATC may vector the followed aircraft, complicating the intercept. This would be far easier to do in a non-radar environment over open water than over land. –  casey Mar 18 at 11:42

Fighter aircraft routinely fly close to give a false indication of numbers (I used to do it), but close formation is rarely necessary. The basic resolution angle formula for a radar is 1.22*wavelength/diameter = angular resolution in radians (circular antenna). For long range Air Traffic Control, typical wavelength is 0.15m, diameter of aerial is 14m. So at 60nm this gives a distance resolution of about 3/4 of a mile. Real antenna have various physical and electronic devices to improve this, so in practice staying at less than 1/10 of this, say 150m, usually means no detection. 150m is the standard FAA specification for its long range radars. We used to use about 75m since we liked to stay "numbers disguised' till about 30nm range. The real problem for the pilot of MH370 would be that he has very little overtake available to effect the intercept, or a suitable Air Interception radar, which makes it highly unlikely he could have achieved it, as he had no fighter background. There used to be a prohibition on HF radio ops during aerial refueling but I believe this was for static electric reasons rather than transmission interference. Flying in close formation (5m or less separation) can cause buffeting effects, but basically a plane 75m away is not going to give any indication it's there. Formation flying is not easy, especially in an airliner!

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for the HF radio bit, it has been adressed here –  Federico Jun 18 at 17:28

I think this is more possible in theory than in fact.

One radar station has given blind spots, but most nations use a whole array of stations. So if you were not seen on one sensor you'd probably be seen on another. Plus shadowing behind another jet would likely put you in a zone of dangerous wake turbulence.

Over the ocean it's easier not to be noticed since ATC radar stations are built on land.

Aside from sensors noticing you dilligence is required by those who monitor the sensors. Seems like that might have been absent on the Malaysian jet, if it actually diverted course radically and crossed the Malaysian peninsula as is being suggested in the news.

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Theoretically possible once you get close enough. In fact military aircraft do it all the time, making a group of smaller aircraft look like a single larger one.
But there's a catch, getting close enough in the first place, and without being noticed.
The crew and passengers of the other airliner would notice a T7 flying in close formation, and even if they didn't see it, they would notice the buffeting and certainly the proximity alarm on the other airliner would notify the crew, who'd notify ATC, who'd send in fighter aircraft to see what the heck was going on.
And don't forget that it takes rather serious training to get aircraft to fly that close, and close coordination between the aircraft to retain that formation and not crash into one another. All of which would not be in place in your scenario.

So no, it's just another conspiracy theory dreamed up by people who've seen aircraft with crews trained for it fly in close formation at an airshow and didn't know what is involved, just like the idea that the T7 would have flown at wavetop level to avoid radio contact, or diving and weaving through the mountains just over the ground.

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