Every day, thousands of aircraft make perfectly routine landings, roll out, exit the runway and head to the gate. It looks simple but throw in bad weather and problems arise that need distinct technologies to make this aspect of airline and airport operations as safe as possible.
Immediately after landing, an aircraft decelerates through a combination of braking, spoilers and thrust reverse, that combination being decided by the crew on the spur of the moment depending on the prevailing conditions, advice from air traffic control (ATC) and personal experience.
There is one big unknown in all of this: the condition of the runway and its braking efficiency. Airports carry out regular checks with vehicles to assess the type of contamination and its depth, as well as measuring surface friction, but these are relatively infrequent as operations cannot be disrupted and are usually more concerned with spotting debris. Another drawback of friction measurements, according to the NTSB, is that they are useful for identifying trends in runway surface conditions but they cannot be used to predict aircraft stopping performance. This is due to the lack of correlation with aircraft braking performance, as this varies between types, and there as variability in design and calibration of the measuring equipment.
Vaisala image.
This means the most regular source of braking information comes from pilots. Unfortunately, not only are these reports entirely subjective, the actual conditions can be disguised by automatic braking systems which aim to provide a steady deceleration. Often, it is only at low speed that pilots can get a feel for actual runway conditions, by which time they will have travelled a fair proportion of its length.
In addition, weather changes constantly, often producing different braking conditions in different parts of the runway. The most obvious threats to adhesion are snow and ice. At least snowfall is consistent but the thawing process can produce random patches, some of which may be icy. Similarly, rainfall is usually constant but a heavy downpour, such as in monsoon conditions, can overcome runway drainage and result in standing water, with additional risk of aquaplaning. A fast moving squall line crossing the airport can even change the braking characteristics for the next aircraft on approach. Finally, blowing sand, while usually a visual problem, can also affect braking.
As can be seen, this makes braking action a worldwide problem. In fact, according to Airbus, statistics show that, between 1999 and 2019, runway excursions (off the side and off the end) accounted for 36% of hull losses and 16% of fatal accidents. In addition, ICAO says runway safety is one of its top three safety priorities, having started looking at the problem as far back as 2004, before setting up a Friction Task Force in 2008. From November 2021 (delayed by a year because of COVID-19), it will be introducing a new Global Reporting Format (GRF) for runway surface conditions. Although ICAO-registered international airports are required to comply, it is expected most other domestic or regional airports will also adopt GRF, although the preferred assessment methods and technologies are likely to vary according to climate, funding, and the amount of traffic.
GRF has two main components: the Runway Condition Assessment Matrix (RCAM), and the Runway Condition Code (RWYCC). The RCAM (see Table 1 above) has assessment criteria derived from a set of observed runway surface conditions and pilot reports of braking action. These are used to set the RWYCC for each third of the runway. This is supposed to help subsequent pilots to identify where contaminants are located and to be prepared for a possible change in aircraft performance. However, as those conditions can be masked by automatic braking, reports for the first two sections could be inaccurate. Any noticeable change may only be detected when there is already a problem.
Despite the limitations of the reports, they are now the primary means for reporting runway conditions, rather than friction measurements. One GRF requirement that helps slightly is the airport must carry out a physical assessment whenever landing aircraft indicate there are significant changes occurring, rather than periodically.
It would seem that a technological solution might be better and, fortunately, help is at hand.
Runway Assessment Condition Matrix
One ground-based company that has responded to GRF is Vaisala in Finland. Well known for its airport meteorological systems, it has produced the Mobile GRF/TALPA Reporter. Vehicle mounted, this combines its well established MD30 sensor originally designed for use on snowplough with the Mobile RCR App and Road AI program.
GPS data automatically detects each runway third while the sensor uses three lasers to three lasers to detect and assess whether conditions are dry, frost, slush, wet or ice. At the same time, the RoadAI pavement data management, visualization and analysis platform uses computer vision and converts raw video data into color-coded condition maps for detailed analysis and tracking of defects in the runway surface. This allows the driver to focus on a visual inspection, looking for foreign objects or debris.
A high speed processor produces a report in GRF format soon after the end of the run, showing the condition of each third, complete with a RWYCC number. It also includes the contaminant type(s) for each runway section along with average and maximum coverage area and depth for each contaminant. After reviewing the report, the inspector is able to modify or confirm the data before sending it by email or SMS to appropriate airport department. The data is also stored by RoadAI for later analysis, with the option of video storage as well.
While that takes care of the assessment when conditions change, the real need is for technology that is aircraft based and available through the landing roll. One answer has come from Airbus.
The company has been working for years on improved braking system. First was the Runway Overrun Prevention System (ROPS), fitted to A320 Family, A330 A350 and A380. This continuously monitored the aircraft’s speed against the remaining runway length, calculating if it could stop in time. In 2018, NAVBLUE, an Airbus company, signed an agreement with Honeywell to provide a combination of ROPS and SmartLanding. This expanded the coverage envelope as SmartLanding, a software enhancement to Honeywell’s Enhanced Ground Proximity Warning System, provides initial warnings if an approach is too high, too fast or is not configured properly for landing. Unstable approaches are another major cause of overruns.
For the A380, a refinement was Brake To Vacate (BTV), which combines position data from aircraft’s GPS and the airport database in the On-Board Airport Navigation System (OANS) as well as Auto-Flight and Auto-Brake facilities. The crew pre-select their preferred exit point and, after landing, BTV controls the deceleration to reach the turn off at a safe speed. Although designed primarily to reduce the amount of heat generated by in the brakes, it does have a secondary role in preventing runway excursions, rather than overruns.
The next step was the Braking Action Computation Function (BACF) for the Airbus A320 Family. BACF development started in 2015, with trials with a number of airlines starting in 2017. Following its release in 2019, users now total 15, including airlines in Europe, North America and India. In all 200 aircraft are currently fitted with BACF, with another 800 or so to be modified in the future.
It uses an aircraft performance model, which contains reference runway conditions, and compares it to the conditions being experienced in each landing, taking into account the degrees of aerodynamic, thrust reverse and brakes being used. It also incorporates the FAA’s Take-off and Landing Performance Assessment (TALPA) Runway Condition Assessment Matrix (RCAM). After the aircraft reaches a ground speed below 30kts, the runway state is displayed to the pilot on a dedicated page on the multifunction control-display unit (MCDU). This gives the crew an opportunity to check their gut feeling about the landing and report to ATC if they feel conditions are different to those advised on approach.
The system proved itself during the trials in snowy weather at a Scandinavian airport. The initial coverage was just 2mm of wet snow, which gave Runway Condition Code (RWYCC) 4 (see Table 1), or Good to Medium conditions. Over the next 35 minutes, reports from four aircraft equipped with BACF showed a deterioration to RWYCC 2 (Medium to Poor). Five minutes later, with increasing snowfall, the runway condition worsened to RWYCC 1 (Poor). This would not have been possible without BACF.
Olivier Donchery, runway safety specialist at NAVBLUE, says that the system continues to prove valuable. About 90% of pilots who have evaluated BACF say it provides relevant information, with slippery when wet conditions detected condition in different region of the world. Airlines have been able to use this objective data during safety meetings with airports.
Having established the basic BACF module, the next step was to develop a system that could also be accessed by airports, airline operational centres and ATC. With RunwaySense, launched in July this year, the data calculated by BACF is sent automatically by ACARS message to NAVBLUE, where it is displayed on a web-service platform. The software is available free of charge to airlines by Airbus and NAVBLUE as part of a general safety drive, says Charles Thornberry, Head of Sales Airport & Airspace at NAVBLUE. It can be selected during the aircraft definition process or retrofitted as an Airline Operations Centre (AOC) application onto the Air Traffic Service Unit (ATSU). The only condition is that operators share the ACARS messages through the platform.
The non-airline users can have access on a subscription basis, he adds, either using the RunwaySense web-service platform or integrated into their existing systems using an API. The important aspect for them is that sharing reports in real-time will allow them to better understand how the conditions are changing, across an individual runway or across the airport as a whole. If there are airports on the network that are susceptible to conditions that might cause excursions, they can be kept under surveillance. Combined with wind, temperature and humidity data, it provides an insight into when snow clearance or runway deicing teams may have to be launched, if schedules are likely to be affected and if separation distances on approach may have to be extended. Of course, the system also provides notice of thawing and dying conditions as the RMYCC numbers change in a positive manner, again allowing users to consider what measures to take.
The more BACF-equipped aircraft are at a particular airport, the richer the data available and Thornberry says RunwaySense is mainly aimed at the narrowbody market, because of the huge fleet size and the range of airport types that they serve, potentially generating huge amounts of data. He expects the Airbus A220 to be added in the future, however, there is some interest for the A350, which has also been certified.
Perhaps the most important aspect of RunwaySense is indirect. The data may be collected by airlines operating Airbus aircraft but dissemination of that data helps all operators with any type aircraft to operate safely in adverse conditions. He says that is part of the company’s drive for overall safety improvements in the industry. And, of course, that matches perfectly with the aims of the ICAO GRF.