The Wet Runway Trap: An Insidious Challenge
August 21, 2017 Credit: Transportation Safety Borad of Canada
WestJet Flight 588 WJA588 — a Boeing 737-6CT with 112 persons on board — ran off the departure end of a landing runway at Montreal/Pierre Elliott Trudeau International Airport (CYUL) and into a grass area during a downpour. No one was injured, nor was the aircraft damaged. However, the incident shows that runway contamination can be quick and insidious. Canada’s Transportation Safety Board (TSB) found that pilots, in general, may want to rethink their approach to wet runways.
The aircraft had departed Toronto/Lester B. Pearson International Airport (CYYZ) on June 5, 2015, on a scheduled flight to CYUL. The captain was the pilot flying (PF) and the first officer was the pilot monitoring (PM). While en route, the PM uploaded the applicable standard arrival procedure and the Runway 24L approach into the FMS before completing the approach briefing. The calculated landing distance obtained from the ACARS was 7,784 ft. assuming flaps set to 30 deg. and the autobrake system set to 1. The PF planned to exit the 9,600-ft.-long runway at the far end to reduce taxi time to the arrival gate. The crew set up the landing configuration based on the ACARs numbers.
During descent, the crew obtained ATIS information Lima, issued at 1418, which stated: wind, 240 deg. at 8 kt.; visibility, 15 sm in light rain showers; broken towering cumulus clouds at 4,500 ft. AGL, another broken layer at 7,500 ft. AGL and overcast at 24,000 ft. AGL; temperature, 23C; dew point, 16C; altimeter, 29.91 in.; ILS Runway 24L and ILS Runway 24R; VFR Runway 24L. Based on this information, the PF planned to carry out a visual approach to Runway 24L with the ILS approach as backup.
While being vectored for the approach, the crew observed moderate to heavy rain activity north-northwest of the field on the aircraft weather radar. The radar displayed heavy rain on the approach path, but no turbulence or hail as the aircraft turned left base leg for Runway 24L. By 1453, when the flight was approximately 8.8 nm from the runway, the pilots had configured the aircraft for landing — flaps at 30 deg., landing gear extended and speedbrakes armed.
At 1455, WJA588 called the tower controller to advise that the aircraft was established on the ILS for Runway 24L. Shortly thereafter, controllers cleared the flight to land and told the crew to expect to exit at the end of the runway. The wind reported to the crew was 350 deg. at 17 kt. with gusts to 22 kt. At that time, the aircraft was flying through heavy rain showers. The wipers were selected ON.
Investigators retrieved the following information from FDR:
The ILS approach was coupled with the autopilot and autothrust (A/T) engaged. The A/T was in SPEED MODE, and the initial selected speed on the mode control panel (MCP) was 130 kt.; however, the crew dialed up the speed to increase to a final value of 140 kt. while the aircraft was descending through 740 ft. AGL. The recorded Vref was 125 kt.; therefore, the final selected speed was Vref + 15.
The crew disengaged the autopilot as the aircraft descended through approximately 280 ft. AGL. The aircraft began to deviate above the glideslope and crossed the threshold at 52 ft. AGL at a speed of 145 kt. (Vref + 20) at 1457:48.
Ten seconds later, the aircraft touched down on its right main landing gear about 2,550 ft. beyond the threshold at a speed of 133 kt. The speedbrakes automatically deployed. The aircraft briefly bounced after final touchdown, and, at 1458:01, the autobrakes activated and both thrust reverser levers were brought to idle detent.
At 1458:08, at a speed of 103 kt., with 4,940 ft. of runway remaining, the PF manually stowed the speedbrakes, which disarmed the autobrakes. Nine seconds later, at 1458:17, the PF applied manual braking; the speed was 92 kt., with 3,320 ft. of runway remaining. Full brake pressure was obtained while the aircraft was at a speed of 85 kt., with 2,270 ft. of runway remaining. At 83 kt., the PF applied maximum reverse thrust. At that point, the PF had steered the aircraft to the right of the runway centerline to avoid the runway end lights and the approach lighting system for the opposite runway.
Full reverse thrust (83% N1) was obtained 10 sec. later. At that point, the aircraft speed was 55 kt., with 550 ft. of runway remaining.
At 1458:43, at a ground speed of approximately 39 kt., the aircraft departed the paved surface of the runway and traveled approximately 200 ft. into the grass before coming to a stop 200 ft. to the right of the runway centerline at 1458:48.
All passengers and crew later deplaned by a mobile staircase placed at the front right door.
Both the captain and the first officer held ATP licenses and current medicals. The captain (PF) had accumulated 9,000 flight hours of which 7,500 were in type. The first officer (PM) had logged 13,898 hr. with 3,360 of those in type. The crew had been on duty for approximately 2 hr. at the time of the occurrence. An analysis of the sleep-wake data showed that neither pilot was in a fatigued state at the time of the occurrence.
What Happened?
Investigators determined the aircraft’s deceleration devices were all functioning properly and that the aircraft had been certified, equipped and maintained in accordance with existing regulations and approved procedures. The crew was experienced and alert, so the TSB focused its analysis on the “management of the operational threats the crew were facing, as well as the use of the available decelerating devices.”
What follows is the TSB’s discussion of the threats facing this crew and, by extension, all crews operating high-performance aircraft on contaminated runways.
Before operational threats can be effectively managed by a crew, said the TSB, they must be identified, and their potential impact — for the current situation and in the future — must be accurately assessed.
When planning the arrival at CYUL, the crew anticipated a visual approach to a wet runway. They expected to roll out and exit the end of Runway 24L.
Guidance material available to assist crews in these situations equates a wet runway with good braking performance, said the TSB. “The landing distance flow chart and stopping distance equivalency tables contained in WestJet’s Flight Operations Manual [FOM] specify that, unless a braking action report [BAR] from a similar type of aircraft has been received to indicate poor or medium braking or the crew expect there to be more than one-eighth inch of standing water on the runway, landing distance calculations for good braking conditions may be used.”
These procedures rely on information that may not always be available to the crew, said the TSB. Based on the Information Lima, which reported light rain, the crew had no reason to expect that the runway would be more than just wet or contaminated by water (more than 3 mm, or one-eighth inch, of standing water). Therefore, in order to obtain ACARS landing distance calculations, the crew selected GOOD in the BAR line. The ACARS calculated landing distance with flaps set to 30 deg. and autobrake set to 1 to be 7,784 ft. Given that this distance included a 15% safety margin and that the runway landing distance available was 9,600 ft., the crew had no reason to reconsider their decision to use a higher autobrake setting or use a flap setting of 40 deg. for landing.
Although the friction index values were within the Canadian standards, said the TSB, it is likely that viscous hydroplaning occurred when the aircraft was approaching the end of the runway, as shown by the lack of deceleration once maximum braking was applied. Combined with the existing downslope, this reduced the possibility of stopping on the runway.
Runway 24L is a textured runway and, as such, provides channels for water to escape and increases braking coefficient in wet conditions. However, according to the FAA Safety Alert for Operators (SAFO) 15009, experience has shown that wheel braking on a grooved runway is degraded when the runway is very wet, and operators should therefore consider being more conservative in their time-of-arrival assessments. If operators do not do so, there is a risk of runway overrun.
Based on the rainfall gauge located at CYUL, only 1.6 mm of precipitation was recorded from 1455 until the aircraft departed the paved surface at 1458. Although the rainfall amount for this period seems small, the rate at which it fell was significant and equivalent to heavy rain. Therefore, the runway was not contaminated, per se, but was likely more than just wet with a shiny appearance. The amount of water observed being sprayed from the aircraft as it was approaching the end of the runway is consistent with some level of water accumulation.
Pilots need timely and accurate runway surface condition information to make correct landing distance calculations, said the TSB. “However, during periods of rain, water depth on a runway may change rapidly; therefore, there is no formal procedure for reporting runway surface conditions as there is with other runway contaminants such as snow and ice. Unless another aircraft has already landed and reported poor braking, the crew have little information with which to develop an expectation of runway performance.
“As shown in this occurrence, the aircraft that had landed on Runway 24L before WestJet Flight 588 [WJA588] had no problems exiting the runway as instructed. However, neither of these aircraft provided a BAR to the controller, nor were they asked to provide one.”
The runway inspection conducted about 18 min. before WJA588 landed did not raise any issues that could have prevented safe landings. However, the runway inspection was carried out before the beginning of moderate and heavy rain showers.
“The fact that rain or even heavy rain is occurring will not automatically prompt a crew to anticipate poor braking, because a wet runway is expected to provide good braking action and the runway is assumed to be adequately drained,” said the TSB. “Unless reports of standing water on the runway are received, pilots are unlikely to consider rain or even heavy rain as threats to their ability to stop the aircraft. This expectation is supported by information contained in the TC AIM, which states that ‘the well-drained runways at most major Canadian airports seldom allow pooling of sufficient water for hydroplaning to occur.’ Therefore, the crew’s initial plan for the arrival, using autobrake setting 1 and thrust reverser to provide minimal deceleration, was consistent with existing guidance that a wet runway should provide good braking action.”
Once on the approach, said the TSB, the crew became aware that weather conditions were worse than anticipated. They noted heavy precipitation on the aircraft’s weather radar and flew through heavy rain on their final approach. “The knowledge that precipitation was intensifying at the airport did not prompt the crew to expect that the runway could be contaminated rather than just wet, and, as a result, they continued to expect good braking performance on a wet runway.”
The TSB points out that the FAA issued a SAFO in August 2015 following an analysis of landing performance in a number of runway overrun occurrences suggesting that the accepted assumption that a wet runway will allow for good braking may not adequately mitigate the risks. The SAFO stated that the braking coefficient of friction was significantly lower than expected for a wet runway and warned operators that 30% to 40% of additional stopping distance may be required on runways that are wet but not flooded.
Given that information, said the TSB, “operators should be more conservative when making landing distance assessments in situations where moderate or heavy precipitation is occurring on non-grooved or non-porous friction course [PFC] runways and where heavy precipitation is occurring on grooved or PFC runways.
If procedures and guidance do not prompt flight crews to anticipate less-than-good braking conditions on wet runways, then there is a risk that landing distance and aircraft management will be inadequate to provide for safe stopping performance.”
Stabilized Approach
As all pilots know, a stabilized approach provides the basis for a good landing. In this occurrence, said the TSB, all the criteria stated in the WestJet FOM for a stable approach were met; however, the aircraft still overran the runway. Government data collected and analyzed from 2003 to 2010 show that 68% of landing overruns occur after stable approaches.
A review of runway overrun occurrences by Boeing demonstrated that they are typically caused by multiple factors. One of these is the approach target speed. As per the WestJet FOM, the target speed is calculated by adding half of the reported steady headwind component plus the gust increment above the steady wind to Vref. The target speed should not be less than Vref + 5 kt. and should not exceed Vref + 20 kt. A common error noted by WestJet check pilots when this procedure was applied was to add half of the total wind rather than half of the headwind component.
The wind reported to the crew before landing was 350 deg. at 17 kt., gusting to 22 kt. This information did not prompt the crew to consider the tailwind component for their target speed calculation. In fact, following the landing clearance, the target speed in the mode control panel (MCP) was increased from 130 kt. to 140 kt.
In this occurrence, the crew calculated an inaccurate target approach speed and crossed the threshold 15 kt. faster than recommended. Combined with a tailwind and a slightly high flare, this resulted in the aircraft touching down beyond the normal touchdown zone, thus reducing the amount of runway available for stopping. However, it was not sufficient to prompt either crewmember to contemplate a go-around before touching down. “It is likely,” said the TSB, “that the crew were unaware of how far beyond the touchdown zone the aircraft was.” (Distance-to-go markers are not provided at Canadian civil airports.)
The TSB said, “One recommended means of mitigating the tendency to continue with a landing outside the touchdown zone is to include a requirement to brief a definite point, such as a taxiway or physical landmark, at which point a go-around will be initiated if the aircraft is not on the ground. Although WestJet’s normal procedures require a go-around to be initiated if a landing cannot be made in the touchdown zone, there is no requirement for the crew to identify a trigger to help recognize when the aircraft has not landed in the touchdown zone. If pilots do not identify a point at which a go-around should be initiated if the aircraft is not on
the ground, then there is a risk that
the landing will result in a runway overrun.”
the ground, then there is a risk that
the landing will result in a runway overrun.”
Deceleration Devices
“The fact that the crew had not recognized the longer-than-normal touchdown and was expecting good braking even in heavy rain is demonstrated by the handling of the aircraft following touchdown,” said the TSB. “The crew continued to implement their plan to use minimal deceleration because they were expecting to exit at the end of the runway.” Reverse thrust was selected shortly after touchdown, but only idle reverse was selected for most of the landing roll.
Boeing engineering simulations showed the aircraft would have remained on the paved surface of the runway despite touching down 1,050 ft. beyond the normal 1,500-ft. touchdown point if maximum reverse thrust had been used for the entire landing roll with the occurrence speedbrake usage and autobrake set to 1. In this incident, the captain delayed the selection of maximum thrust by approximately 25 sec. after touchdown.
In addition, the captain stowed the aircraft’s speedbrakes above the speed of 80 kt. specified in WestJet’s SOPs, and they were not redeployed during the landing roll. This reduced the normal load on the gear and the aerodynamic drag. Consequently, the deceleration rate decreased, which increased the stopping distance.
The results of engineering simulations run by Boeing show the importance of speedbrakes and their role in stopping the airplane as the runway condition deteriorates. Based on the FDR data, the crew-commanded brake pressure remained approximately at levels commanded by the autobrakes once disarmed; however, the deceleration decreased by one half. According to Boeing, this can be primarily attributed to the stowage of the speedbrakes. However, in this occurrence, even if the speedbrakes had been kept deployed for the entire landing roll, the aircraft would still have overrun the end of the runway given the use of reverser thrust during the occurrence.
Conclusion
As shown by a review of runway overrun occurrences by Boeing, high speed, tailwind, long landing and delayed use of deceleration devices were factors that have significantly contributed to runway overruns.
Fortunately, the runway end safety area (RESA) available at CYUL allowed the aircraft to decelerate in a controlled manner and it sustained no damage.
Suggested best practices for preventing overruns, said the TSB, include conducting a positive touchdown in the touchdown zone and making maximum use of deceleration devices early in the landing roll. This is particularly true of reverse thrust, which is more effective at higher speeds. As shown in this occurrence, the instruction to exit at the end of the runway contributed to the minimal use of deceleration devices early in the landing roll, as the crew was attempting to expedite their exit at the end of Runway 24L.
It is normal for landing aircraft that are going to the terminal to be instructed to exit at the end when landing on Runway 24L at CYUL. Having landing aircraft travel to the end of the runway facilitates traffic flow by leaving the single taxiway free for departing aircraft. In these circumstances, safety and best practices for preventing runway overruns would dictate that flight crews land with full use of available deceleration devices and then proceed to the instructed exit point at a normal taxi speed.
Flight data monitoring conducted by WestJet following the occurrence suggests that non-standard use of deceleration devices is more prevalent on runways where aircraft are typically instructed to exit at the end. In these situations, pilots may be inclined to maintain speed and decelerate at the end of the runway rather than decelerate normally through the early application of all available deceleration devices and taxi the aircraft to the end of the runway. If pilots limit the use of deceleration devices to comply with a real or perceived requirement to expedite exiting at the end of a runway, then there is a risk that the landing will result in a runway overrun.
So, there’s a lot to think about with this incident. The lessons discussed here can be applied to any high-performance aircraft touching down on “wet” runways. In business aviation, the risks can be higher when arriving at non-airline airports. The old instructor’s admonition still holds, I think: “There is nothing as useless as fuel in the truck or runway behind you.”
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