Question regarding error in altitude

[Bill Watson]

So, the fact I've never even examined the deviation reports stapled in my log book is acceptable.

Digital transmissions either provide accurate data or are failed.

Controllers don't convert pressure altitudes to MSL on every transmission like the way they generate magnetic vectors to produce tracks (?)

And the OP needs to go up and try to reproduce the problem to see if his plane has a critical problem that needs fixing or whether ATC made a momentary and unexplained error.

You guys are entertaining. Any advice on how to get the cable guy out of my house?


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[Capt. Geoffrey Thorpe]

And the OP needs to go up and try to reproduce the problem to see if his plane has a critical problem that needs fixing or whether ATC made a momentary and unexplained error.

I would call my avionics guy who can compare the mode C output (and altimeter) to a calibrated pressure sensor in his transponder testing device - then you actually know what is and is not working.

 

[Capt. Geoffrey Thorpe]

After all, it's your own backside that will be strapped to a machine that's hurtling towards the ground in low IMC where the only thing stopping you from crashing might be the altimeter, whose validity is attested to in that report.

If it was my backside strapped to a machine hurtling towards the ground in low IMC, I'm not sure that I would be comfortable trusting a (potentially up to) two year old report on my altimeter.

You guys call ME insane???

 

[RotorDude]

If it was my backside strapped to a machine hurtling towards the ground in low IMC, I'm not sure that I would be comfortable trusting a (potentially up to) two year old report on my altimeter.
You guys call ME insane???

Some (many?) people consider leaving good old terra firma in a single engine piston airplane (or any airplane for that matter) "insanity". So we all have our own levels of acceptable risk taking, and we all try to mitigate it in a reasonable and practical way. Some of us even rely on the government for guidance.

 

[Velocity173]

So, the fact I've never even examined the deviation reports stapled in my log book is acceptable.

Digital transmissions either provide accurate data or are failed.

Controllers don't convert pressure altitudes to MSL on every transmission like the way they generate magnetic vectorso to produce tracks (?)

And the OP needs to go up and try to reproduce the problem to see if his plane has a critical problem that needs fixing or whether ATC made a momentary and unexplained error.

You guys are entertaining. Any advice on how to get the cable guy out of my house?


Sent from my iPad using Tapatalk

The controller doesn't convert the PA to MSL, the computer automatically does it. Like in the pic here, the scope shows 29.73. That's an auto feed from their baro altimeter. In the old days you had a radar flight data guy update it periodically and if someone reported off by 300 ft or more, that's the first thing you looked at. If it hadn't been updated in hours, it could be the culprit.

In the OP's case, I'd be willing to bet the controller got a couple hits of 7,800 but then it went back to 7,700. Hence the reason he didn't tell him "Cessna 345, stop altitude squawk, altitude differs by 300 ft." Or he was just lazy and didn't care.

The whole magnetic track thing isn't a feature that have on all the time. It's something they bring up for an individual aircraft to see it's predicted track. Old school, you had to eye ball it based on target trail.

 

[tsts4]

If it was my backside strapped to a machine hurtling towards the ground in low IMC, I'm not sure that I would be comfortable trusting a (potentially up to) two year old report on my altimeter.

You guys call ME insane???

I like having the report but I use it to give me a baseline of performance but I don't rely on it for flights. Instead I rely on comparison to field elevation as part of my preflight instrument check that all instrument pilots should be performing at some point prior to departure.

 

[bluerooster]

Sometimes I just log into this group for laughs like this.

The ground radar interrogates the aircraft transponder on 1030 MHz. and the transponder replies on 1090 MHz. Splitting the difference and converting to wavelength we can say that this is a 30 cm (28.3 to be precise) signal.

When the transponder replies, it does so with two framing pulses at the beginning and end of this digital string: a series of 12 pulses each 0.45usec wide separated by 1.45usec. Since 2^12=4096, we say that there are 4096 separate squawk codes available. The ident pulse, if it replies, does so 4.35usec after the last discrete code pulse. Another interrogation, another burst of pulses from the transponder (which also flashes the little blinkie green light on the transponder).

If there is an altitude encoder, the transponder immediately replies with another two framing pulses with the same exact information coding system but leaves out the D1 pulse to identify the response as altitude as opposed to discrete code.

Now, rather than take wild a$$ guesses as to how oil on the transponder antenna will magically let the discrete code through yet scramble the altitude code that comes through microseconds later, why not tell me from your technical background how you think this might happen? I've only been designing aircraft electronics for 50+ years, so I sure don't know everything. Tell me, how do you see this happening?

Jim

Apparently you didn't read the entire post. Nowhere did I say that the transmission will become scrambled. just weaker.