| It has always been interesting to me how few pilots really know that the transmitted altitude from both encoding and “blind" encoding altimeters, are always referenced to 29.92 inches of mercury; and therefore are not effected by the pilot adjusting the local pressure setting. This fact came out a few years ago when I participated in an AWACS Red Flag exercise, where I went as a Reservist on the active duty mission planning cell. Even though I was a “staff puke", but still talked them into letting me fly one mission “in the seat” as the Mission Crew Commander; and on that flight I was amazed to see how this Mode C operational issue could cause so many problems for the crew.
The basic issue came about since the E-3 was assigned to orbit “on the range”, vs. in a standard FAA controlled E-3 orbit. Therefore the E-3 pilots had to set and fly their orbit altitude referenced to the “range altimeter”, vs. the setting in 29.92 (upon passing 17,999 ft.). The effect of this non-standard procedure resulted in the E-3's altitude (on our scopes) displaying 200 feet too low (due to the altimeter setting being about 2/10ths higher than referenced 29.92).
So instead of our system displaying the normal “290” on our scopes (for flight level 290), it instead displayed “288”. Frantic calls immediately went up to the flight deck from the Senior Director, who wanted to confirm that they were really flying at 29,000 ft.! The flight deck confirmed this fact, and felt a little miffed that a “back-end guy” would ever think he didn’t know how to fly his assigned altitude, etc. The Senior Director then contacted the Air Surveillance Officer (ASO) and told him the IFF equipment must be messed up, since the E-3’s altitude on the scopes was displaying too low based on the altitude the the pilots were flying at! The ASO said his equipment checked out good, but he would check with the RADAR Tech. to see if he was showing any faults, etc.
From my perspective, it was amazing to see how much confusion this little event caused among the crew, and how much “SA” (situational awareness) it sucked out of the team, at a time when a/c were starting to check in, and everyone needed to be focused on the upcoming mission. Instead they were clogging up the internal voice nets to discuss a MODE C issue that didn’t really exist! It soon became clear that my active duty buddies had flailed long enough, and that they weren’t going to solve the issue on their own, so I had to step in and explain how MODE C worked, and that this was completely normal, and the IFF system was also working as it should! The mission could therefore continue…so let’s get on with it!
Upon landing, our Aircraft Commander still didn’t believe that his manipulation of the altimeter setting in the cockpit, DID NOT have an effect on what was transmitted through Mode C to the FAA centers, etc. He had been taught at pilot training that he could fly below his minimum altitude blocks and keep ATC from knowing it by adjusting his alt. setting. I therefore asked him to perform a simple google search of the FAA's web site; and there he found a couple of articles that explained how Mode C works, and therefore confirmed that what I had already told him was true.
All this just shows that even though we taxpayers spend a couple million dollars training each military pilot; there are still some important bits of information that get overlooked, and aren’t necessarily included in the syllabus… :)
Fly safe out there!
Doug RV-9A / 13B (on-hold for a few more years)
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