Altimeter accuracy?

[Paris Hilten]

Just overflew an A/C on same airway 2,000ft below. TCAS showed -20 as normal. But the radalt showed 1800 ft all the time when overhead. And radalt don't lie.

Can anyone explain this?

[Rainboe]

Don't worry your sweet blonde head about it babe, go back to the disco.

[Paris Hilten]

Nice. Be serious.

[BOAC]

TCAS displays the altitude of all traffic AS READ BY THE altimeter systems. If yours and the other'altimeters were, say, 100 ft out each way there is your difference. I do not know how accurate the rad alt is at 1800' - it is not critical, so it may be in error too.

[Mad (Flt) Scientist]

Also, one might wonder which part of the aircraft was making the radar return - if the aircraft below were a T-tail and the Hstab were the prime return, that's another distance error, since the altimeter isn't reporting where the fintop is.

[Old Smokey]

Assuming that all of the instrumentation was working correctly, it would require an atmosphere at ISA-21.7°C for 2000 feet of True Altitude difference to translate to 1800 feet of Pressure Altitude difference.

Not unusual..........

Regards,

Old Smokey

[bflyer]

old smokey ..... that was very impressive for someomne retarded such as yours truly

[Paris Hilton]

We were at FL400, them FL380. The SAT was around normal (-57C).
And surely the true altitude diff. is what the radalt shows. (subject to the A/C dimensions compared to position of static port, which certainly wouldn't account for 200ft).

[VH-ABC]

Paris,

My equipment doesnt have the luxury of TCAS, and cant remember back to the days of ATPL study... so when you TCAS shows traffic at -20, does it round off to the nearest figure? Maybe this, added to the rest of the possible reasons for discrepancy is where that 200' came from?

[gimmesumviagra]

Old Smokes, I am really impressed.

Rainboe, I am disappointed at your patronising attitude in 2007.

Paris, all I can suggest is a combination of Altitude keeping error and Air Data Computer error acting in unison to compound the error.

Guess if you were in RVSM airspace you would have been twitching a bit more (like me transiting the Indian Sub-Continent & Mid East!!) So Good!

[Blacksheep]

And radalt don't lie.

Hmm. If avionics were that infallible, avionics specialists would all be out of our jobs. Radio Altimeters are pretty good at working out how far is the huge mass of planet Earth below the aircraft. When responding to a target as small as another aeroplane there'd be considerable doubt.

What is interesting is that you got a -20 on TCAS and 1800 feet on the Rad Alt. The TCAS system gets its information from the two aircraft exchanging their barometric altitude data over the Mode 'S' link, so there is one discrepancy to keep avionics people interested, if not very busy. One system is definitely telling porkies...

Pressure altitude transducers aren't perfect but they're all calibrated to the same laws (These days: but it wasn't always so...) You don't give your actual altitude or aircraft type - which would give a clue as to the altimeter in use - but up around 30,000 feet, the certified accuracy of a typical altimeter transducer would be around +/- 300 feet. So, if a lower aircraft is 300 feet above assigned and a higher aircraft is 300 feet below you still have 400 feet seperation in an RVSM airway, or 1,400 feet in non-RVSM airspace.
On, lets say a B767, your 200 feet difference is no cause for concern. Its within the system's RVSM certified limits.

RVSM seperation of 1,000 feet only became practical since the eighties when altimeters became mose accurate. Even then, aircraft still have to be certified as RVSM capable. We can't have any ancient 707 freighters, equipped with pre-historic "Kollsman Integrated Flight Instrument Systems" (KIFIS - anyone remember that?) mixing it with modern aircraft types can we?