KC-135 Landing Altimeter Setting Errors
Beech 200 Super King Air. Photo courtesy of Skybrary.aero
By MR. CHARLES CAMP, AMC MFOQA Program Manager,
and MR. BRIAN GREER, KC-135 MFOQA Flight Data Analyst
“Reach 75, say altitude,”—a request that causes a pang in your stomach followed immediately by a quick look at your altimeter. Before you answer, you have a quick discussion with the crew in the cockpit to confirm that you have the correct altimeter value set—or at least a setting that your crew believes is correct. Sound familiar? Be honest; we have all been there, but maybe you are asking yourself just how pervasive is this issue, and why should we care.
This article will examine Line Operations Safety Audit (LOSA) observations, Airman Safety Action Program (ASAP) submissions, and Military Flight Operations Quality Assurance (MFOQA) analysis to show that misset altimeters is an issue across all Mission Design Series (MDS) in the Mobility Air Forces (MAF) and that the altitude transition level is a major cause for altimeter errors for both climbouts and descents.
Air Mobility Command has conducted LOSA observations since 2010 on almost every MDS currently flying in the MAF. Most aircraft systems have been through the LOSA process multiple times. With no exceptions, altitude deviations were highlighted as a trend item in every LOSA report. For example, one LOSA report stated that primary altimeter setting errors occurred at a rate of 12 percent of observations and that those errors were mismanaged by the crew 78 percent of the time. Furthermore, those errors resulted in undesired aircraft states (UAS) 69 percent of the time. A different MDS LOSA report stated that the observers saw primary altimeter errors on 13 percent of flights with a mismanagement rate of 71 percent. A third MDS LOSA report detailed this error at a rate of 11 percent with a mismanagement rate of 57 percent by the crews of that MDS. Clearly, an outsider riding along on a mission sees misset altimeters as an issue, but what are crews reporting?
The ASAP system provides crews with a tool for self-reporting mistakes and safety issues. Although the rate of ASAP submissions by MAF crews was low in 2009, the number has since increased. From the inception of the ASAP system, MAF crews have reported altimeter-setting issues and those issues most often lead the monthly trends. A quick query of the ASAP database to find where the crew indicated a misset altimeter as the major contributor to an unsafe event was highlighted in 60 submissions. That search included both climbing through and descending past the transition altitude. More important, the error caused the crew to believe that their aircraft was at a higher altitude than it actually was 21 times—that’s more than one-third of the time!
Now I ask you, how many times have you made a mistake and elected not to submit an ASAP—for whatever reason? If you are honest with yourself, the number is high, so in reality, those misset altimeter events may be two (120) or three times (180) more frequent than the current submission rate shows.
Although the ASAP program relies on self-reporting, MFOQA analysis is derived from flight data captured by onboard sensors and retained for future examination. As mentioned previously, misset altimeters have been showing up as issues in several proactive safety programs. To see if MFOQA analysis could find all the incidents not observed by LOSA or reported by ASAP, the MFOQA analysts and software developers set out on a quest. Unfortunately, that quest proved very difficult because the transition altitude can vary per location; however, members of the MFOQA team focused their investigation on the landing phase and developed an ingenious trigger in the MFOQA software to determine when a crew landed with an incorrect altimeter setting. The analysis compared digital aeronautical flight information file touchdown zone elevation (TDZE) data with the recorded aircraft-captured pressure altitude at touchdown. When the difference was more than 150’ (high or low), the trigger fired. This trigger is available now in the KC-135R/T and the C-17, but the programmers are adding the trigger to other MAF MFOQA-capable MDSs as an MDS’s software is updated.
In addition, analysts and programmers are investigating a way to include transition altitude analysis capabilities. The figure below provides an example of the KC-135 analysis.
The blue areas of the bars show altimeter discrepancies of 150’ high or greater, and the gray areas show discrepancies of 150’ low or greater (with high meaning that the aircraft is higher than the pilot thinks it is and low meaning that the aircraft is lower than the pilot thinks it is). Why is this analysis important? If a pilot descends to an altitude that is 200’ higher than is shown on the approach plate, the crew is not at the decision height and may proceed using incorrect information. The opposite (that is, a lower altitude) is even worse, for the aircraft would be below the altitude on the approach plate, which negates all obstacle protections and may result in a controlled flight into terrain (CFIT).
Following is an example of one of the high instances at an international airport. The approach plate shows the TDZE as 1,125’; however, the pressure altitude captured from the aircraft at touchdown showed 946’ with an altimeter setting of 30.13, for a difference of 179’. In addition, the Meteorological Terminal Air Report for the time of the approach showed an altimeter setting of 30.31, validating the difference.
In a worst-case scenario, the crew would go-around early on a low-visibility approach at 200’ height above airport (HAA). The crew would see 1,146’ at 200’ HAA (on the altimeter with the wrong setting), but because the decision height (DH) is 1,325’ (which the crew would have seen earlier), the crew would go-around. An ASAP submission described how the crew followed that scenario exactly and elected to go-around because the field was not in sight. After catching the altimeter error, the crew then flew the same approach and landed.
So, what’s the big deal? According to SKYbrary (an aviation safety site created by the International Civil Aviation Organization and the Flight Safety Foundation), several accidents and incidents have occurred wherein the incorrect altimeter setting was either a cause of or a contributing factor in an altitude-level deviation or CFIT/near CFIT, as in the following example:
BE20/SF34, vicinity Stornoway UK, 2011. On 31 December 2011, a USAF C-12 Beech King Air descended 700 feet below the cleared outbound altitude on a procedural nonprecision approach to Stornoway in uncontrolled airspace in IMC [instrument meteorological conditions] and and also failed to fly the procedure correctly. As a result, it came into conflict with a Saab 340 inbound on the same procedure. The investigation found that the C-12 crew had interpreted the QNH [sea-level pressure] given by air traffic control (ATC) as 990 MB [millibars—Hector Pascal] as 29.90 inches, the subscale setting units used in the United States. The Saab 340 pilot saw the opposite-direction traffic on TCAS [traffic collision avoidance system] and descended early to increase separation.
The C-12 crew was 670’ lower than they thought they were! From MAF ASAP submissions, we know of a C-17 crew that misset their altimeter by 500’ high, a KC-135 crew that left 29.92 in their altimeter and were 400’ low throughout their approach, and a C-21 crew that thought they heard the controller tell them to set 29.97 inches when the controller had actually said 997 MB, which resulted in the aircraft being 500’ lower than the crew thought.
So, why are these errors happening? The Air Force Instruction (AFI) 11-2 Mission Design Series (MDS) Volume 3 guidance requires the pilot monitoring (PM) to make altimeter callouts during climb and includes a requirement for both pilots to state the altimeter setting during descent. Unfortunately, LOSA observations show that the most frequent callout error was the omission of the transition altitude call, resulting in a 40 percent error rate. In those instances in which an omitted transition callout led to a primary altimeter setting error, the end result was a UAS 95 percent of the time.
What’s the moral of the story? Utilize solid crew resource management or cockpit resource management and PM skills to ensure that you have the right setting in your altimeters. Review the details of other examples of misset altimeters provided by other crew members at the ASAP Scoreboard (https://afsas.safety.af.mil/). Finally, keep an eye on the monthly MFOQA products stored in your electronic flight bag—and Fly Safe!