Using Aircraft as a Sensor on Contaminated Runways

In the world of commercial jets, it is well known that Runway Excursions (RE) are one of the top three causes of accidents.

Accident statistics show that RE caused 35% of hull-losses and 14% of fatal accidents between 1998-2017. Given this status, Airbus and other manufacturers are investing in development of technology to reduce RE accidents.

Product features such as Airbus’ ROPS (Runway Overrun Prevention System) are already in service and providing real time energy and landing performance monitoring information to flight crews.

However, accurate knowledge of the runway condition is also key for the validity of landing performance computations, and a clear case can be made for the need to improve pilot awareness of runway surface conditions.

Indeed, national Safety bodies including the NTSB of the USA and the UK AAIB have identified the need to develop “an operationally feasible airplane-based braking ability / runway surface condition measurement and communication system”. 

Today, there are typically three methods available by which runway surface conditions are evaluated:

• Runway contaminant type and depth observations
• Ground surface friction measurements
• Braking action reports from pilots

Contaminant type and depth observations are in general physically conducted by airport personnel on the runway surface. The conditions are assessed through a combination of visual observations and spot-checks. However, it can be a difficult task to consolidate what may be differing conditions across the entire width and length of the runway into a succinct runway condition report.

In addition, during active precipitation and/or freezing/melting conditions, the validity of the information may become outdated soon after it is issued.

Ground surface friction measurements provide a more qualitative approach to taking measurements along certain points on a runway. However, as noted by the NTSB, they are useful for identifying trends in runway surface condition ut are not recommended for use in predicting aircraft stopping performance.

This is due to the lack of correlation with aircraft braking performance, as well as variability in equipment design and calibration.

While the airport operator is responsible for generating the Runway Condition Codes for a runway, pilots are responsible for providing accurate braking action reports. Indeed, providing braking action reports is a significant role that pilots play in preventing runway excursions for all airplanes.

Braking action reports contain the pilot’s assessment of the manner in which an aircraft responds to the application of wheel brakes. The latest terminology for these reports is identified by rulemaking activity from the ICAO, the FAA, and the EASA, and is explained in Table 1.

Under these new rules, which are expected to become the applicable worldwide standard in November 2020, pilots will be required to radio braking action reports to ATC whenever they are requested, or if the pilot has assessed braking action is less than previously reported. ATC will be required to relay information to airport operators, and depending on the situation, to other pilots in approach.

The forthcoming rules also define the expected response from airports if runway surface conditions deteriorate enough that two consecutive reports of ‘Poor’ conditions are received. In this case, the airport will be expected to re-assess the runway conditions. Additionally, If “Less Than Poor” braking action is reported, the runway will be closed to further operations until the airport operator can improve the runway’s condition.

These reports thus play an important part in the cycle of runway surface condition assessment and reporting.

Aeroplane deceleration results from several forces: aerodynamic drag forces, generated by the airframe and in particular the ground spoilers; reverse thrust, if available; wheel braking.

In general, a braking action report should characterize the availability (or lack thereof) of wheel braking. The difficulty for a pilot is in differentiating in realtime, which portion of the total deceleration is coming from the wheel-brakes.

This difficulty is compounded by the typical use of autobrakes on contaminated runways. As the autobrake commands an overall airplane deceleration rate, the pilot is able to detect a lack of wheel-braking when the target deceleration is not achieved, however it is still difficult to differentiate how much each component is contributing to the deceleration.

Once the aircraft decelerates to lower speeds (generally below 60kt), pilots often use manual braking and at these speeds the aerodynamic drag and reverse thrust forces are negligible. It is often in this zone where pilots are able to more easily “feel” the runway by using the brake pedals to understand the braking action.

Given these complexities, making an accurate report can be a difficult task for a pilot, and braking report quality can become subject to differences of subjectivity between different pilots. To resolve this and provide objective and consistent braking action reports, Airbus has developed technology which uses aircraft data measured during the ground run to identify the available braking action.

Airbus has been developing a new aircraft function to address the need identified by the NTSB and other national aviation Safety bodies, for “an operationally feasible airplane-based braking ability / runway surface condition measurement and communication system”.

The implementation of this function on Airbus aircraft is called the “Braking Action Computation Function’ (BACF)”.

The fundamental principle of the function is, post landing, to use the data measured by the aircraft during its deceleration roll to identify the braking action level. By using the aircraft performance model, it is possible to differentiate the part of deceleration coming from either aerodynamic, thrust reverse, or wheel-braking.

Subsequently, by comparing the actual wheel braking performance to models of wheel-braking performance under different “reference” runway conditions, the runway state which most closely resembles the experienced deceleration is determined.

Additionally, using GPS data available from the aircraft navigation systems, it is possible to identify which section of the runway the aircraft is on when a runway state is identified. The function can identify several states at different points on the runway.

A few seconds after the aircraft speed has decreased below 30kts, details about the runway state become available to the pilot on a dedicated MCDU page (fig.1).

If the pilot felt that the runway was slippery, or in a different condition to that communicated by Air Traffic Services (ATS), this information can be accessed by the pilot and radioed to ATS at an appropriate moment.

As shown in (fig.2), in addition the information available to the pilot through the MCDU, the data calculated by BACF is also sent automatically by ACARS message to NAVBLUE.

NAVBLUE will collect and display the results on a web-service platform called RunwaySense. The users of this service are expected to include airports, airline operational centres, and air traffic control.

This technological approach is similar to the various mobile traffic applications which share traffic data in real-time to allow drivers to see and avoid traffic jams.

Indeed, the goal of this new Airbus-NAVBLUE technology is to provide a platform where airspace users are sharing reports in real-time to better understand how the runway condition is trending, and to allow the airport to anticipate and mitigate slippery conditions. The more aircraft which participate in the sharing, the better the real-time map of conditions becomes.

Users of the service will be able to view runway condition information across a whole airport, or, as shown in (fig.3), on an individual runway. Airport level information will provide a high level status of the airport across the different runways, whilst runway level information will enable users to check runway condition trends versus different climatic conditions such as winds, temperatures and humidity.

BACF and the RunwaySense collaborative web-platform are integrated as part of the overall RunwaySense Service from NAVBLUE.

The operational & safety benefit comes from sharing the data. To maximise the facilitation of the information, Airbus & NAVBLUE decided to make the BACF software Free of Charge (FOC) provided that airlines share the BACF ACARS messages through the RunwaySense platform.

The BACF software consists of an ATSU AOC application and will be available as an Airbus Service Bulletin.

All airlines which contribute will have basic access to the RunwaySense web platform where they can visualize and track all the BACF reports sent by their aircraft. Airlines can also choose to receive additional information about flight conditions at key airports in their route network.

For airport and ATC users, access to the NAVBLUE RunwaySense web platform will be possible through a paid subscription.