ATC just cleared you to descend to 13,000 feet over the mountains of Colorado. You read back 10,000 feet and they don't catch the mistake. What now?
Jet Descending Into Aspen
While many articles have been focused on pilot reports, NASA ASRS reports are filed by air traffic controllers too. In this example, a controller didn't catch an incorrect descent readback from a crew flying into the mountains around Aspen. Here's what happened...
Working as the "Controller in Charge" for the operation, I was over-viewing the operation on both sides of the tower cab. The approach controller was dealing with an aircraft operating 10 miles out on the localizer at an altitude unfavorable for a visual approach. While deciding to re-sequence another aircraft back around, the approach controller had issued a descent to the second aircraft with a bad readback due to what I believe was expectation bias. We will call the aircraft with a missed readback "Aircraft X."
I also did not hear the incorrectly assigned altitude. The approach controller issued a descent to 13,000 feet and the pilot read back 10,000 feet. He was not reissued the correct altitude to maintain. The attention of the controller and myself was taken away from Aircraft X while we dealt with another IFR arrival and a VFR aircraft in the pattern. The approach controller observed Aircraft X descending through 12,400 feet (below the MVA for the area) and issued a low altitude alert and instructions to climb immediately.
Aircraft X responded and reported field in sight requesting the visual approach. The approach controller issued the visual clearance to the requested runway. Aircraft X landed safely with no issues. With relatively clear weather, this mistake did not result in disaster. However, under the right conditions, this could have been an extremely dangerous situation with high terrain surrounding Aspen.
MVA: Minimum Vectoring Altitude
An MVA is the lowest altitude that ATC can vector you around a particular section of airspace. Approach and Center controllers can divide their scopes into small sections of airspace, separating obstacles and terrain from areas with lower vectoring altitudes. When radar coverage is available from an approach or center facility, MVAs are a great way to get as low as possible over a VFR airport. But unfortunately, MVAs aren't published on your IFR charts.
Here's what the MVA looks like around Aspen's mountainous terrain...
You can, however, find MVAs published online. Click here for FAA MVAs and MIAs. Keep in mind, these drawings aren't easy to decipher and unless you're an ATC controller, and figuring out where an airport is located on an MVA chart can be difficult.
Contributing Factor: Expectation Bias
Most of the time, pilots read back clearances correctly. And if either party doesn't hear the readback clearly, they ask for it to be repeated. Since ATC was accustomed to aircraft following instructions as-stated, there likely wasn't a pressure to double-check that the pilots were following the proper clearance. Controllers are far more familiar with the minimum IFR altitudes around their particular airports than the pilots flying in and out.
Expectation bias is a real threat to all of us in aviation, whether we're pilots or air traffic controllers. When we expect to hear or see something, our minds have a tendency to trick us into thinking we've accomplished the stated goal.
What Can You Do?
In this case, the best thing the pilots could have done was cross-check their descent clearance with the height of terrain below or immediately around them. Since MVAs aren't easily accessible, there's not much else you can do to ensure ATC gives you a correct instruction.
As for listening skills, pilots and controllers need to pay extra attention to descent clearances around terrain. If either party misunderstands or doesn't clearly hear the readback, they should ask for clarification. If you start to feel external pressure, slow down to cross-check your work. The more pressure you're under, the more likely you are to make a mistake.
Has This Happened To You?
Communications are challenging and don't always have outstanding quality in busy airspace. Have you ever misheard or noticed an incorrect clearance? Tell us about your experiences in the comments below.
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