As winter fog tightens its grip on Dhaka, flight diversions from Hazrat Shahjalal International Airport have once again become a regular occurrence, disrupting schedules, straining airline operations and causing widespread passenger suffering.
Over the past week alone, at least 50 flights were forced to land at alternative airports, including Kolkata, Chittagong and Sylhet, after runway visibility in Dhaka dropped below safe limits.
At the heart of the problem lies a technical limitation: Dhaka’s main airport is currently operating under a downgraded Instrument Landing System (ILS), reducing its ability to handle aircraft safely during dense fog.
Why fog forces flights to divert
Dense fog significantly reduces runway visibility, particularly during late-night and early-morning hours in winter.
When pilots cannot visually identify the runway or rely adequately on landing guidance systems, aircraft are not permitted to land for safety reasons.
In such cases, pilots either hold the aircraft in the air in the hope that visibility improves or divert to a nearby airport where conditions are better.
Air traffic officials say that at present, Hazrat Shahjalal International Airport can operate only under ILS Category I conditions, which require a minimum runway visibility of around 1,200 metres.
When visibility drops below that threshold, landing becomes unsafe, forcing aircraft to divert.
What is ILS and why it matters
The Instrument Landing System is a critical navigation aid that helps pilots land safely during poor visibility by providing precise lateral and vertical guidance to the runway.
The International Civil Aviation Organization (ICAO) classifies ILS into different categories based on how low visibility can be while still allowing safe landings.
ILS Category I allows landings with visibility typically no lower than 1,200 metres.
ILS Category II permits safe landings with visibility between 500 and 750 metres.
ILS Category III (A, B and C) allows landings in extremely low or near-zero visibility but requires highly advanced technology and extensive infrastructure.
While many major international airports operate with Category II or III systems to handle fog, experts say Category III is costly and technically complex, making it difficult to implement quickly in countries like Bangladesh.
Why Dhaka was downgraded
According to airport and aviation officials, Hazrat Shahjalal International Airport previously had ILS Category II capability.
However, it has been downgraded because it no longer meets all mandatory technical requirements.
To maintain Category II status, three key conditions must be met simultaneously:
The ILS equipment itself must be certified to Category II standards.
At least 95% of the runway lighting system - including approach lights, centreline lights and edge lights - must be fully functional.
Meteorological instruments measuring runway visual range must operate accurately at all times.
Officials say the airport is currently failing to meet the second condition.
The effectiveness of the runway lighting system has dropped below the required 95%, making it impossible to operate safely under Category II rules.
Civil Aviation Authority of Bangladesh (CAAB) Assistant Director (Public Relations) Muhammad Kausar Mahmud said a Thai Airways aircraft damaged part of the runway lighting system during landing on October 29.
“Since then, the airport has been operating under Category I. These lights are specialized, expensive and need to be imported. Tenders have been called, and installation will be completed soon,” he said.
Where do diverted flights go?
When flights cannot land in Dhaka, pilots divert to nearby airports with better visibility and adequate facilities. Chittagong and Sylhet are common domestic alternatives, while Kolkata is frequently used for international flights due to its proximity and infrastructure.
Transit passengers are the worst affected, as diversions often lead to missed connections, overnight delays and unexpected travel costs.
Aviation expert and former Biman board member Kazi Wahidul Alam said the crisis reflects a lack of seasonal preparedness.
“Category II is not just about installing equipment. It requires constant monitoring, routine maintenance and rapid repairs. If the lighting system had been fully inspected and upgraded before winter, much of this disruption could have been avoided,” he said.
Impact on passengers and airlines
Passengers report hours of uncertainty, with aircraft circling in the sky before diversion decisions are made.
Many complain that information about diversions is often delayed, adding to anxiety and confusion.
For airlines, diversions mean additional fuel consumption, crew rescheduling, airport handling charges and knock-on delays for subsequent flights.
“One diversion disrupts multiple flights across the network,” said a private airline official.
“If this continues throughout winter, operational pressure increases significantly.”
CAAB says work is underway to restore the damaged lighting system and reinstate Category II operations.
Until then, fog-related diversions are likely to continue during peak winter conditions.
Experts warn that unless long-term planning improves, including preventive maintenance before winter and clearer passenger communication, Dhaka will continue to face the same cycle of disruption every fog season.