Interesting ASRS Re Altimeters

[rocketflyer84]

The following NASA report (465074) is quite interesting, both because of what happened but also because the person that wrote it seems to not have understood what really happened here even though all the evidence is right there in the text.

What really happened here was the airliner in Class A had the altimeter at 29.92 while in his report the VFR pilot says he was at 17,900 ft with an altimiter of 29.68. This lower altimiter setting means the pilot really was flying at or very close to FL185 as reported by the airliner. ATC is reading mode C that's hardcoded to 29.92 so would also see FL185 as reported.

I WAS PERFORMING AERIAL MAPPING OF HWY 24 FROM PETERSON AFB TO CALHAN. THE ALT I WAS TRYING TO MAINTAIN FOR THIS PROJECT WAS 17900 FT MSL INDICATED. MY PLAN FOR THE FLT WAS TO REMAIN CLR OF CLASS A AIRSPACE AND AVOID FILING AN IFR FLT PLAN. I HAD RECEIVED COS LCL ATIS INFO AND SET MY ALTIMETER TO 29.68. WHILE FLYING THE PROJECT FLT LINE DOWN HWY 24, I NOTICED MY INDICATED ALT WAS REACHING FL180 AND PROMPTLY CORRECTED FOR 17900 FT MSL TO AVOID CLASS A AIRSPACE. THROUGHOUT THE WHOLE PROJECT, I WAS NOT IN COM WITH ANY CTLING AGENCY. I WAS MONITORING COS APCH. AFTER COMPLETING THE HWY 24 PROJECT I LEFT THE AREA AND DSNDED TO THE N FOR MORE WORK IN THE CENTENNIAL AREA. I CONTACTED APA TWR FOR CLRNC INTO THEIR AIRSPACE. CENTENNIAL SWITCHED ME TO 123.7. I CONTACTED THE TWR AND GAVE THEM THE INFO FOR MY SURVEY FLT AND WAS TOLD TO 'PROCEED AS REQUEST.' APA TWR CALLED FOR AN IDENT AND GAVE ME A PHONE NUMBER TO CONTACT ZDV. UPON LNDG, I CONTACTED THE WATCH SUPVR AT ZDV. HE INFORMED ME THAT I WAS OPERATING AT FL185 IN THE COS AND CAME WITHIN 100 FT OF AN AIRLINER. I TOLD HIM THAT I WAS ATTEMPTING TO REMAIN CLR OF CLASS A AIRSPACE AND SUSPECTED MY MODE C WAS IN ERROR. THE SUPVR TOLD ME MY MODE C WAS FINE BECAUSE THE IDENT AT APA WAS CORRECT, AND THE AIRLINER COULD VISUALLY TELL THAT I WAS AT FL185. THROUGHOUT THE FLT, MY INTENT WAS TO REMAIN CLR OF CLASS A AIRSPACE. IN THE FUTURE, I PLAN TO FILE AN IFR FLT PLAN FOR ANY PROJECT OVER 17000 FT MSL TO AVOID ANY POSSIBLE AIRSPACE VIOLATION.

 

[eetrojan]

Interesting. You may be right that he didn't get the overlap issue. While it probably wasn't super wise, I think it was legal because he was technically below 18000 MSL.

Looking around, in fact, it looks like FAR 92.121 tries to account for the fact that somebody using the current altimeter setting may actually be flying at the same altitude as aircraft that are using 29.92 by establishing a "lowest usable flight level" based on the current altimeter setting.

 

[Sluggo63]

Interesting. You may be right that he didn't get the overlap issue. While it probably wasn't super wise, I think it was legal because he was technically below 18000 MSL.

Looking around, in fact, it looks like FAR 92.121 tries to account for the fact that somebody using the current altimeter setting may actually be flying at the same altitude as aircraft that are using 29.92 by establishing a "lowest usable flight level" based on the current altimeter setting.

This is pretty common, and transparent to us in the US. The controllers are supposed to know and apply these Transition Levels to aircraft that are operating at the bottom end of the Class A.

Interestingly, most countries outside of the US and Canada actually have a Transition Altitude and a Transition Level listed and more often than not, they are not the same altitude/Flight Level.

For instance, Taiwan has a Transition Altitude of 11,000 feet, and a Transition Level of FL130. So between 11,000 and FL130, they can not assign you an altitude.

 

[Van Johnston]

My question would be why was he planning to fly at 17900 instead of17500? And why wasn't the airliner at FL180 or FL190?

 

[eetrojan]

My question would be why was he planning to fly at 17900 instead of17500? And why wasn't the airliner at FL180 or FL190?

Good point. Under the VFR altitude rules, I suppose he should have planned either 16,500 or 17,500, not 17,900.

If my math is right, if an altimeter setting of 29.68 showed 17,900 MSL, increasing the setting to 29.92 would make it indicate 18,140, or "FL181.4".

The FAR table above shows the lowest usable flight level that day should have been FL185.

 

[rocketflyer84]

Interesting. You may be right that he didn't get the overlap issue. While it probably wasn't super wise, I think it was legal because he was technically below 18000 MSL.

Looking around, in fact, it looks like FAR 92.121 tries to account for the fact that somebody using the current altimeter setting may actually be flying at the same altitude as aircraft that are using 29.92 by establishing a "lowest usable flight level" based on the current altimeter setting.

Yep... And had the VFR pilot been at 17,500 (highest altitude he should have been flying at) there would have been a lot more separation.

Not sure what he was thinking with this 17,900 business.

 

[rocketflyer84]

If my math is right, if an altimeter setting of 29.68 showed 17,900 MSL, increasing the setting to 29.92 would make it indicate 18,140, or "FL181.4".

Under perfect standard conditions yes... But he says he was "drifting to 18,000" so that's another 100 ft, he may have not quite tuned in 29.68 exactly so there's some error there plus the Mode C only reports to ATC in 100 ft increments so theres another 100 ft gap so it's entirely probable that ATC saw FL185 on their scope.

 

[OverTQ]

Good point. Under the VFR altitude rules, I suppose he should have planned either 16,500 or 17,500, not 17,900.

I'm guessing that he was maneuvering for the surveying, not in "level cruising flight".

 

[eetrojan]

I'm guessing that he was maneuvering for the surveying, not in "level cruising flight".

Maybe, but the ASRS report quoted in the OP says "THE ALT I WAS TRYING TO MAINTAIN FOR THIS PROJECT WAS 17900 FT MSL INDICATED. "

 

[rocketflyer84]

I'm guessing that he was maneuvering for the surveying, not in "level cruising flight".

He said: "THE ALT I WAS TRYING TO MAINTAIN FOR THIS PROJECT WAS 17900 FT MSL INDICATED." (Sorry for the all caps just pasting)

That sounds an awful lot like he was cruising at 17,900. Not sure what he was thinking there, or why he thought buzzing 100ft below a big no-go zone for VFR was a good idea.

 

[StinkBug]

17900 was the first thing that jumped out at me too. Dunno why you'd purposely put yourself right on the edge there even if it were an acceptable altitude. Just like all the guys that like to fly 100' under a Class B shelf. Drift 30' high and have a little error in your transponder and you're busted.

 

[MAKG1]

17900 was the first thing that jumped out at me too. Dunno why you'd purposely put yourself right on the edge there even if it were an acceptable altitude. Just like all the guys that like to fly 100' under a Class B shelf. Drift 30' high and have a little error in your transponder and you're busted.

I fly 100 under Class B because the floor is 1500 and it's over the Bay, and there is a bridge in there as well, and a Class C airport's (KOAK) longest runway.

And yes, I have had an incident where the transponder broadcast 200 feet higher than I really was. Not anywhere near the issue you're assuming.

"Cessna 123XY say altitude." "Altitude one thousand four hundred 3XY." Done.

It's no more than irritating.