REFLECTOR: Static port calibration

[Kent Murley]

I've hit up Center technicians for info since the FAA doesn't even  
teach us controllers about encoders. Approach Controls are similar but  
here's roughly how it works in Centers.

The altimeters we use in the cockpit are independent of the altitude  
encoders which send data to the transponder. Some autopilots with  
altitude hold link up to our old pressure sensors to save money but  
I'm just talking about the steam gauges here.

As a pilot, it always felt like we had altitude encoders that pilots  
can't touch so that controllers wouldn't get fooled by lying pilots.  
Actually, the encoder is a backup system that lets the current local  
pressure be adjusted by the Center ground systems in case the  
adjustable altimeter in the cockpit fails.

The altitude encoder in the plane sends altitude data to the  
transponder as if the entire atmosphere were standard, locked at 29.92  
in.

Each Center's map is split into 'radar sort boxes' that are about 30  
miles on a side. Each of these sort boxes is assigned, via software, a  
certain weather reporting station to use its local pressure setting.

The radar processor at each Center takes the standard altitude data  
from the plane's transponder, adjusts it with the current barometric  
data for the sort box the plane is flying through, and displays it to  
the controller in the nearest hundred feet.

There are always backhoes cutting through phone lines between the WX  
station and the Center making the controller ask you to get the ASOS  
or ATIS yourself. When the Center doesn't get current WX data the  
correction factor the Center has soon becomes old and the altimeter in  
the cockpit has no accurate backup, though I've never seen the encoder/ 
ground system method be off more than 800 feet after 8 hours of old  
data. (Gotta love the new GPS altitude data)

Each method backs up the other most of the time, but legally, the  
pilot stating what their barometric altimeter shows is the final  
authority. Kohlsmann is my hero.

Inexperienced controllers, when the two methods don't agree, will say,  
"Say altitude". Most of us pilots would want to be honest and say  
"five thousand eight hundred", thinking our lives depend on it.

Experienced controllers, when the two methods don't agree, will say,  
"Verify level at six thousand", knowing this is more likely to elicit  
"Affirmative" from the pilot which means no paperwork for either of us.

For instrument approaches and when one traffic pattern underlies  
another our calibration becomes important. Upon further questioning  
I'm sure the FAA will deny I work for them.

Kent Murley
XLRG near Seattle



On Mar 4, 2009, at 10:16 AM, <aminetech at bluefrog.com> <aminetech at bluefrog.com 
 > wrote:

> If ATC is seeing the altitude transmitted by your transponder,  
> naturally it will be identical, assuming you are reading from the  
> same instrument that feeds the transponder.  Am I missing something?
>
> John
>
> --- ALVentures at cox.net wrote:
>
>
>
> I’ve always found my altitudes consistent with where ATC says I am –  
> so I don’t think I need to change anything.
>
>
> Al
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