Tower Developmental Controller receiving training on combined positions and Local Controller providing OJT described a potential conflict when an airport vehicle misunderstood a 'expect' clearance and entered the runway without clearance.
Synopsis
Tower Developmental Controller receiving training on combined positions and Local Controller providing OJT described a potential conflict when an airport vehicle misunderstood a 'expect' clearance and entered the runway without clearance.
Narrative
At the time of the incident; I had been training on the Local position combined with Ground and Flight Data for around 40 minutes; with having worked alone on Ground for about an hour prior to the training session on Local Control. I currently have around 30-35 hours of training time on Local Control. My facility's target hours are 250. For around the past two hours; various airport vehicles such as a snowplow (Airport X); a 'brush' vehicle (Airport Y); and an airport SUV (Airport Z); had been checking the braking action and clearing the runway and taxiways of snow. Airport X was the command vehicle and had been on the airport surface the longest. Continuously throughout this period; the vehicles were repeatedly cleared on the runway surface and told to vacate in the anticipation of arriving and departing aircraft; without incident. The visual approach to Runway 13 was in use. Towards the end of my session and the occurrence of the incident; a King Air was cleared for the Visual Approach by the Departure Controller; and then switched to the Local Control frequency prior to reaching a wide left midfield downwind for Runway 13. Shortly after King Air was cleared to land on Runway 13; a Baron (BE58) had landed and was rolling out on the runway. Airport X and Airport Y were holding short of Runway 13 at the approach end of Taxiway AZ. It was at this time I told Airport X; 'Expect three minute delay for arriving aircraft' Airport X immediately read back 'Airport X proceeding on Runway 13.' I do not recall hearing this transmission but heard it clearly when I listened to the tape. Immediately after Airport X's transmission; I told the Baron; 'Turn left next taxiway; remain this frequency;' as the aircraft was still rolling out. There was a brief moment of silence on the radio; two seconds or less; when my trainer noticed Airport X proceeding on the approach end of Runway 13. My trainer immediately came over the frequency and told Airport X; 'Airport X; negative; you are not cleared on the runway; vacate Runway 13;' Immediately followed by; 'King Air; cancel landing clearance; vehicle on the runway.' King Air read back 'Roger' and informed the Tower that he had the vehicle on the runway in sight. King Air was on a wide midfield left downwind at this point. Airport X did not transmit further until perhaps Z-12 seconds later when he transmitted; 'Airport X off and holding short Runway 13.' At this point; my trainer then re-cleared King Air to land. I do not think it was more than 5-7 seconds between Airport X proceeding on the runway and my trainer telling him to vacate. After my trainer re-cleared the King Air to land; I resumed control of the position and issued a departure clearance for a commercial jet on the terminal ramp. Several more minutes passed; perhaps four minutes; before the King Air landed safely and without incident; and the aircraft's approach did not need to be altered to create further separation. I do not feel as though this incident created an unsafe or imminently dangerous situation for the vehicles or the aircraft involved; given their positions; but this same incident would have created a very dangerous situation if the King Air had been on a short final instead of a wide left downwind. One factor that had may have lead to the problem is how long Airport X had been on the field removing snow. During a two hour period of constantly being cleared and holding short of the runway; the vehicle operator may have become mentally exhausted or complacent from the extensive hearing of clearances from the Tower. Several times during this two hour period; Airport X was cleared on the runway immediately following a departing or arriving aircraft. At the time of the incident; Airport X may have been expecting a clearance on the runway as he saw the Baron roll out in front of him; and assumed he was being cleared on the runway when I told him to expect a three minute delay. I may have also made the same mistake in expecting Airport X to read back that he was holding short; which may be why I don't recall him transmitting that he was proceeding on the runway. Allowing or requiring airport vehicle operators to take a break similar to the way air traffic controllers do; may prevent a similar incident. Using some form of physical memory aid in an airport vehicle; the same way controllers do; may help them to better keep track of their clearances on the airport surface. I also feel as though I would like to improve my scan of the runway surface during future hours of my training; as my trainer observed the airport vehicle on the runway before I did. Also; I do not feel as though the practice of informing vehicles who are holding short how long of a delay to expect is safe. If no transmission is made to the vehicle; I would think it would significantly reduce the likelihood of a runway incursion.
Second reporter narrative
Providing OJT on Local Control/Ground Control combined. [We had] snow removal in progress; three vehicles holding short of Runway 13 for landing traffic; in anticipation of clearance back on the runway to continue work. Regional jet [was] rolling out down field [and a] King Air on left downwind; midfield; next to land; and cleared to land. Developmental told Airport X to expect 3 min delay due to additional inbound aircraft. Airport X replied; 'Roger; proceeding on Runway 13 [or something very close to those words];' and immediately crossed the hold-short line and proceeded to the far edge of the runway. Developmental missed the bad read back; and issued another unrelated transmission. Even a prompt correction on the part of the developmental would not have prevented the incursion. When I realized the developmental wasn't going to follow up; I over-rode him and corrected the vehicle; telling him to vacate the runway. I also canceled the King Air's landing clearance. Vehicle vacated the runway; and I re-cleared the King Air (still on downwind/base) to land. Recommendation; given that the vehicle started moving as it was giving ATC an incorrect read back; perhaps nothing ATC could have done would have prevented this incursion; including a prompt correction of the bad read back by either the developmental or myself. It was a classic case of a vehicle operator hearing what was expected; instead of what was actually said. It was; by the way; also an excellent teaching moment for the developmental; as the proximity of the arriving aircraft allowed a comfortable interval of time for me; as OJTI; to correct the problem; and for the gravity of the situation to 'sink in.'
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.