The transponder "spiked"

[eastern wiseguy]

The scene....pressure fairly low (990) and the initial clearance was to climb to 3000 feet on the qnh 990. The aircraft appears on radar passing 3300 feet(mode charlie as yet unverified)reaches 3400 feet then descends to 3100 feet whence the aircraft checks in at 3000 feet. Atco .(me)verifies mode c and with the aircraft identified instructs climb to FL090. In a quiet moment with all traffic off the frequency I ask the crew about the level "excursion".I am informed that they were level at three thousand feet but that because of a high energy departure the mode charlie has spiked. In 28 years I have never heard of this. I am concerned because if this is not a spoof then the potential for TCAS TA/RA is large. What do you think? Spoofing(and treating me like an idiot :hmm: ) or a genuine phenomenon?

[Alex Whittingham]

Spoofed, I would think. Of course the Mode C transmits pressure altitude information to the nearest 100ft not the QNH-based altitude. Do you have a filter on your display which converts the output to altitude using the QNH of the day?

[eastern wiseguy]

Alex...thanks that was the feeling of myself and my equally experienced colleague on Approach. We do have the qnh input automatically and we are both aware of the reporting limitations of the system.

As an aside I am giving a presentation on level busts as part of the Belfast ATCO/PILOT forum on 17th November in the Park Plaza BFS .I look forward to exchanging views there.

[411A]

Don't specifically know the transistion altitude at the specified location, but if it is rather low, the ground equipment may well be at fault.

Many locations in Europe/UK still insist on using low transistion altitudes, and clearly need to rethink the idea of using a higher one.

Will they?
Not likely any time soon, I'm afraid.
Old habits die hard.

OTOH, the crew might well have climbed higher than cleared, and if the rather low transistion altitude was the cause, the local ATC unit might well ask why.