Confidential Human Factors
Incident Reporting Programme
The Charity
Aviation
Maritime
ATC861
Initial Report
Report text (summary of original)
It’s becoming very common that arrival slots into EGLL (Heathrow) are being put down as destination airfield weather slots, when in fact the weather is completely fine.
Example (one of many):
CTOT 0645, TTO: PTID LOGAN, TO 0829 FL F249, DELAY: AW/015.
METAR: EGLL 300520Z AUTO VRB02KT 9999 BKN011 OVC038 12/11 Q1028 NOSIG
TAF: EGLL 300456Z 3006/3112 VRB03KT 9999 BKN040 PROB30 3006/3009 7000
Below is [a description] of the issues and why I believe they have the potential to affect safety, crew operations, and overall trust in the slot allocation process. ATC communications or subsequent operational updates suggest flow-management constraints reason rather than meteorological factors.
Nature of the problem – In my submitted example, the assigned delay code AW/015 cited weather as the primary factor, but both the METAR and TAF indicated light winds, good visibility, and only insignificant cloud. Such discrepancies between the stated cause (weather) and actual MET conditions appear to occur regularly—perhaps several times a month, if not more.
Operational and safety concerns
Fuel and Flight Planning:
– Impact: If flight crews consistently see arrival slot delays attributed to weather—even when it’s evidently not a factor—it can lead to confusion about how best to plan fuel loads or how to anticipate potential holding or diversion scenarios.
– Safety Relevance: Proper situational awareness is crucial for decision-making in flight. Misleading “weather” attributions may cause crews to over- or under-fuel and misjudge potential risks.
Crew Fatigue and Disruption:
– Impact: Frequent last-minute changes to Calculated Take-Off Times that are labelled as weather delays can result in operational inefficiencies, extended on-ground or airborne holding, and potential crew fatigue.
– Safety Relevance: Fatigue is a well-known risk factor in flight safety. Unplanned or poorly explained delays can disrupt crew rest cycles and degrade alertness.
Trust and Transparency:
– Impact: The integrity of slot allocation systems – and by extension, trust in ATC and airport operations – depends on honest reporting. Consistently citing weather when it is not the genuine cause undermines that trust.
– Safety Relevance: A culture of openness is essential for a healthy safety environment. When front-line personnel perceive inaccuracies or obfuscations, they may be less inclined to report genuine issues or anomalies.
Conclusion and request
I hope this expanded explanation clarifies the nature of my concern: the frequent misattribution of arrival slot delays to weather, especially when conditions are demonstrably benign. I believe this practice, whether intentional or due to system limitations, could indirectly impact flight safety, operational efficiency, and trust within the aviation community.
CHIRP Comment
The nub of the report is that ATC issued slots were being imposed for weather but, when pilots arrived at the destination, the weather was fine and that such slots were causing delays and inefficiencies due to imposed flow restrictions. CHIRP has consulted experts and been informed that such weather slots can be imposed when there was a ‘prob xx’ forecast of bad weather, or when dedicated Met staff at major airports predict a risk of low visibility. Airports will prefer to account for potential bad weather even though it might not materialise. This means that they are postured with pre-planned slot times and therefore ready to continue operations if the forecast weather occurred. For the early first waves, this decision is often based on weather forecasts published the evening before, and so it is often the case that the actual weather might be different come the morning. The decision to invoke slot times involves use of software analysis of weather data which includes historical data from previous similar weather patterns and whether there have been restrictions applied before.
NATS have provided some information on slot times and weather patterns on their website although it is acknowledged that the intended audience is not experienced aviation professionals such as the reporter of this CHIRP report. NATS also provided to CHIRP a breakdown of the decision making regarding the imposed flow rates leading up to, and on, the day the reporter quotes. ATC and airport capacity regulations are labelled as such; only those that are due to forecast poor weather, or a risk of low visibility, will be classified as weather regulations.