My Fifth Mistake as an Air Traffic Controller

On 23-11-2011, in ATC world, by jim

Yes, here I am again. You’d think by now I would stop this self-flagellation, but this is not for me. I record these incidents so you may see the mistake and avoid the same or similar in your life. I noted in my last missile that pride was a key ingredient in most of my mistakes. So it was and is.

Altimetry, a simple system; Know the pressure of the atmosphere and you can accurately judge distance above the surface. But we humans have made it a bit more difficult than stated. We have different methods of measurement. Some measure in inches others measure in centimeters. Compounding this is the insistence of some to measure height above sea level and others above the ground level. In the parlance of the time QNH and QFE.

Because of these anomalies the controller at Rhein-Main in 1957 had to have available the QNH and QFE in both Inches of mercury and Millibars of mercury. This means four numbers. The field elevation at Frankfurt International Airport was 272 feet Mean Sea Level. Therefore a QNH reading of 29.92 inches becomes a QFE of 272 feet less, or 29.65 and the concomitant millibar numbers, 1012.3 and —–.
Each hour when the weather observer recorded the observation on a Dimiphone recording, the QNH and QFE would be given in both inches and millibars. Those numbers would then be written on a backlit Plexiglas placard and posted so everyone in the control room could see the placard.

For those who are interested, the QNH and QFE three letter groups are from the days of Morse code transmission of information. They are from the list of “Q” signals. QDM is the magnetic course to a station, QSY is, “Change your radio frequency to xxxx“. There is a long list of these abbreviations. Many were still used as shorthand phraseology in radiotelephony in the 50’s and 60’s, especially in the international aviation system.

With all that as preface, this is the incident as it happened:

Air France 242 was inbound for landing. Frankfurt International was utilizing runways 07L/R. At the time there was no precision approach aid to either of those runways. The time was night, but a beautiful one with the stars shining brightly and visibility over 10 miles. A vector to field in sight was all that was needed for most pilots. This was the procedure desired by the pilot of Air France 242.

The latest weather observation and altimetry was posted on the weather board. I had copied the information myself even though I was working the Approach Control position and my Data Assistant normally would do the job.

The normal pre-landing information was broadcast to Air France 242 and because I knew he would ask, I gave both QNH and QFE in millibars. A request from the pilot for a confirmation of those numbers was made and I glanced to the board and read the numbers again.

Well, on short final the pilot made a comment about the altimetry again and asked that the numbers be verified. Because of the insistence of the pilot I looked more critically at the set of numbers. Damn, there was an error.

When I copied the numbers I had somehow written the QNH number in the QFE place and therefore when the pilot, by instruments, was at ground level his altimeter would indicate he was still 272 feet above ground. Luckily, it was a beautifully clear night and the pilot had been able to adjust his descent visually. In the Air France cockpit procedures, one pilot had the QFE set in his altimeter and the other the QNH.
The pilot made a report of this error and we had a recording review session the next day. It was obvious what I had done wrong and therefore after a proper review and explanation there was nothing else to do. Whether a report was made with my name attached or an entry into my record, I don’t know.

Again, my mistake could be attributed to carelessness and pride. Pride that I had done the job correctly the first time. Confidence is good, but checking it twice is better.

Auf Weidersehen. BAKER SUGAR

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  1. Mike Smitten says:

    I think you mean millimetres of mercury (mmHg) since millibars of mercury makes no sense. A millibar is a unit of pressure equal to 1 Hectopascal

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