Have you ever misunderstood or misread a clearance? Here's how one mistake nearly led to a CFIT accident...
Report: ATC Misses Incorrect Altitude Readback
We found the following NASA ASRS report published about a near CFIT (controlled flight into terrain) accident with a jet flying under IFR. Here's what happened...
I was piloting a Cessna Citation in the south part of ZZZ1 Class B airspace. I was on an IFR flight plan and was in communication with Approach Control. From somewhere a few miles west of ZZZ2, I was told to turn to a heading of north and descend to 2,500'. After a short amount of time, I started to look at my charts thinking that was too low for my position.
I recall the controller being busy, trying to reach an aircraft that did not respond, and was awaiting an opportunity to confirm my altitude, when the controller called my tail number and told me to immediately climb to 3500' as I was in the middle of antennas. I later listened to the tape, and ATC assigned us to 3,500', but I understood 2,500', and repeated back 2,500' twice. Despite saying 2,500' twice, the controller did not catch it.
Trust, But Verify
ATC keeps you clear of terrain and obstacles, especially under IFR. But controllers are human too, and they can make mistakes. In this case, both the pilot and ATC made a mistake. The pilot misheard the altitude, read it back incorrectly, and ATC didn't catch the mistake. In a busy radio environment, thoughts and calls can get jumbled.
As the PIC, this means you need to "trust, but verify." If you're close to the ground, cross-check the ATC instruction with navigation charts to see your altitude above the terrain. In this case, the pilot only realized how low they were flying once they were already well below the assigned 3,500 feet.
If The Radios Are Busy, Wait
Unless you have an "immediately" or "expedite," you don't need to rush your climb or descent. If the radios are busy and you can't get a word in to verify the instructions, stay at your current altitude. It's better to stay where you are, then assume you've heard correctly and try to get clarification after a mistake has been made.
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 threat to all of us in aviation, whether pilots or air traffic controllers. When we expect to hear or see something, our minds have a tendency to convince us that we've accomplished the stated goal.
Have you ever been confused by an ATC clearance? Tell us in the comments below.
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