Finally, the moment you have been waiting for has arrived. Approach has just issued descent instructions, and a heading to fly to intercept the ILS localizer.
The terminal area is busy with traffic, and instrument meteorological conditions prevail, which is probably why you received a series of vectors and descents until 23 nautical miles from your home airport.
Now, as you prepare to level off from the descent, a ground proximity warning goes off.
What’s the correct response? Check for system malfunctions? Request an altitude check? Double-check your clearance? Go missed—even this early in the approach?
You can check the system later; you can ask ATC all the questions you want once you resolve the alert. So go missed.
Confronted with this scenario, an air carrier flight crew executed the missed approach. Upon notifying ATC, the pilots quickly discovered what had gone wrong.
“I reported this to the controller and she said that our clearance was to descend at 15 DME to 2,900 feet,” one of the two pilots wrote in a filing with the Aviation Safety Reporting System.
The pilot, who was familiar with the local area’s higher terrain but had been “distracted by a combination of other actors,” added a procedural critique: “This was a non-standard clearance that we’re not accustomed to hearing in the terminal area, especially on final. Normally when lower altitude instructions are given the pilot is expected to begin descent, or if on final to maintain the assigned altitude until glidepath intercept on an ILS.”
How close did the flight come to becoming a controlled flight into terrain (CFIT) accident?
Unknown, but some familiar ingredients were present. “The inability of controllers and pilots to properly communicate has been a factor in many CFIT accidents. Heavy workloads can lead to hurried communication and the use of abbreviated or non-standard phraseology,” cautions Chapter 3 of the Instrument Procedures Handbook.. It describes another instance of a misunderstood instruction—in this example, a flight crew reading back a mistaken descent clearance which goes uncorrected by the controller, resulting in a ground proximity alert, and another close call.
As the publication points out, high workloads and tricky weather elevate the risk of error and miscommunication. That’s when a pilot’s situational awareness remains a last defense, even during a phase of flight in which responsibility for terrain clearance still rests with ATC.
Dan Namowitz
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