Westjet B736 at Montreal on Jun 5th 2015, overran runway
Last Update: May 16, 2017 / 21:13:29 GMT/Zulu time
Incident Facts
Date of incident
Jun 5, 2015
Classification
Incident
Airline
Westjet Airlines
Flight number
WS-588
Departure
Toronto, Canada
Destination
Montreal, Canada
Aircraft Registration
C-GWCT
Aircraft Type
Boeing 737-600
ICAO Type Designator
B736
Airport ICAO Code
CYUL
The airline confirmed the aircraft exited the runway onto grass upon arrival. All passengers and crew were safe however.
Canada's TSB have opened an investigation.
On Jun 11th 2015 the TSB reported that the aircraft overran the end of the runway by 75 feet/23 meters. There were no injuries, all on board disembarked via mobile stairs at the right front door. The TSB have dispatched two investigators on site.
On May 16th 2017 the TSB released their final report concluding the probable cause of the runway overrun was:
Findings as to causes and contributing factors
- 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 crew calculated an inaccurate target approach speed and crossed the threshold 15 knots 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.
- The captain delayed the selection of maximum thrust by approximately 25 seconds after touchdown. Consequently, the distance required to stop the aircraft increased.
- The captain stowed the aircraft¡¯s speedbrakes above the speed of 80 knots specified in WestJet¡¯s standard operating procedures, 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 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 were attempting to expedite their exit at the end of Runway 24L.
- 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 downslope, this reduced the possibility of stopping on the runway.
Findings as to risk
- If operators do not consider making more conservative time-of-arrival assessments when active heavy precipitation exists, then there is a risk of runway overrun.
- If there is no specific guidance on how to assess and report runway surface conditions during non-winter months, then there is a risk that crews will be unable to properly assess landing conditions.
- 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.
- 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.
- If crews are not provided with clear clues to indicate how far from the end of a runway they are, then there is a risk that deceleration devices will not be used in a timely manner to prevent a runway overrun.
- If pilots limit the use of deceleration devices to comply with a real or perceived requirement to expedite exiting at the end of the runway, then there is a risk that the landing will result in a runway overrun.
- If Code 4 runways do not have a 300 m runway end safety area or a means of stopping aircraft that provides an equivalent level of safety, then there is a risk of injuries to occupants in the event of a runway overrun.
Other findings
- 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.
- The pilot flying intentionally disarmed the autobrake by retracting the spoilers at 103 knots and no manual brake application occurred until 16 seconds after touchdown. Therefore, it is reasonable to believe that the pilot flying judged that there was sufficient runway remaining when the spoilers were retracted, based on the observed runway condition.
The TSB reported the captain (ATPL, 9000 hours total, 7500 hours on type) was pilot flying, the first officer (ATPL, 13,898 hours total, 3,360 hours on type) was pilot monitoring. The crew performed a visual approach to runway 24L with the ILS approach as backup. The crew observed moderate to heavy rain northnorthwest of the field while being vectored for the approach, established on final the crew observed heavy rain on the approach path. The aircraft was configured for landing with gear down and flaps 30 8.8nm before the runway threshold. Tower cleared the flight to land advising to vacate the runway at the end and reporting winds from 350 degrees at 17 knots gusting 22 knots. The aircraft entered heavy rain, the wind screen wipers were set on.
The crew had computed a Vref of 125 KIAS and set their Vapp at 140 KIAS (Vref+15). Descending through 280 feet AGL the autopilot was disconnected, the aircraft began to deviate above glideslope and crossed the threshold at 52 feet AGL at 145 KIAS. 10 seconds later the aircraft touched down first on its right main gear about 2550 feet past the runway threshold at 133 KIAS. The speedbrakes deployed automatically, the autobrakes activated 3 seconds later and both thrust reverser levers were brought to the idle detent.
With 4940 feet of runway remaining, at 103 KIAS, the captain stowed the speed brakes, which also disarmed the autobrakes, the captain applied manual brakes. The aircraft slowed through 85 KIAS with 2270 feet of runway remaining, at 83 KIAS the captain applied maximum reverse thrust and steered the aircraft to the right of the runway center line to avoid the runway end lights and the approach lights for the opposite runway. The engines had accelerated to full revserse thrust after 10 seconds, speed was 55 KIAS about 550 feet before the runway end. The aircraft departed the paved surface of the runway at a ground speed of 39 knots and travelled approximately 200 feet into the grass before coming to a stop about 200 feet to the right of the runway center line.
There were no injuries, all passengers deplaned through a mobile stair at the right hand front door. The aircraft sustained no damage but left traces of up to 12 inches deep in the soft ground.
With respect to expectation of runway conditions the TSB analysed:
These procedures rely on information that may not always be available to the crew. Based on the information obtained from ATIS 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 ¨û inch, of standing water). Therefore, in order to obtain aircraft communications addressing and reporting system (ACARS) landing distance calculations, the crew selected GOOD in the BAR line. The ACARS calculated landing distance with flaps set to 30¡Æ and autobrake set to 1 was 7784 feet. Given that this distance included a 15% safety margin and that the runway landing distance available was 9600 feet, the crew had no reason to reconsider their decision to use a higher autobrake setting or use a flap setting of 40¡Æ for landing.
The TSB analysed with respect to the actual runway condition:
Although the friction index values were within the TP312 standards, 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. It is normal to have some rubber contaminant buildup and texture loss near both touchdown zones of a runway even after rubber removal, especially when the runway is wet. Combined with the downslope, this reduced the possibility of stopping on the runway.
...
Runway 24L longitudinal and transversal slope gradients fall within the established standards of TP312 and nothing indicates that runway drainage was inadequate. 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 according to the criteria in Environment Canada¡¯s Manual of Surface Weather Observations (MANOBS). Therefore, the runway was not contaminated, per se, but was likely more than just wet with a shiny appearance. Furthermore, 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.
With respect to approach, flare and landing the TSB analysed:
A review of runway overrun occurrences by Boeing demonstrated that a runway overrun is typically caused by multiple factors. One of these factors 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 knots and should not exceed VREF + 20 knots. 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¡ÆM at 17 knots, gusting to 22 knots. This information did not prompt the crew to take into account 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 knots to 140 knots.
In this occurrence, the crew calculated an inaccurate target approach speed and crossed the threshold 15 knots 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 crew member to contemplate a go-around before touching down. It is likely that the crew were unaware of how far beyond the touchdown zone the aircraft was.
With respect to managing deceleration the TSB analysed:
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. 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. According to WestJet¡¯s standard operating procedures (SOPs), idle reverse may be used when required to comply with noise abatement requirements; otherwise, normal or maximum reverse is to be used, depending on the stopping performance required and landing performance data provided to crews.
As shown by engineering simulations run by Boeing, if maximum reverse thrust had been used for the entire landing roll with the occurrence speedbrake usage and autobrake set to 1, the aircraft would have remained on the paved surface of the runway despite touching down 1050 feet beyond the normal 1500-foot touchdown point. In this occurrence, the captain delayed the selection of maximum thrust by approximately 25 seconds after touchdown. Consequently, the distance required to stop the aircraft increased.
In addition, the captain stowed the aircraft¡¯s speedbrakes above the speed of 80 knots 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 flight data recorder (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 used during the occurrence.
Metars:
CYUL 052052Z 32003KT 270V360 15SM FEW010 OVC065 20/18 A2991 RMK SF1SC7 SLP128 DENSITY ALT 700FT
CYUL 052000Z 34004KT 280V010 12SM FEW020 BKN050 OVC075 20/18 A2992 RMK CF2SC3AC3 SLP131 DENSITY ALT 600FT
CYUL 051913Z 31005KT 270V330 3SM -SHRA BR FEW011 SCT025TCU OVC050 19/18 A2993 RMK SF1TCU3SC4 VIS SE 11/2 SLP134 DENSITY ALT 500FT
CYUL 051906Z 31008G18KT 2SM SHRA BR FEW011 OVC025TCU 19/18 A2993 RMK SF2TCU6 PRESRR SLP135 DENSITY ALT 500FT
CYUL 051900Z 31012G23KT 1 1/2SM +SHRA SCT011 OVC022TCU 19/18 A2993 RMK SF3TCU5 PRESRR SLP134 DENSITY ALT 600FT
CYUL 051851Z 33018KT 250V340 15SM SHRA FEW012 BKN025TCU OVC075 20/18 A2992 RMK SF2TCU5AC1 VIS SW-N 21/2 PRESRR SLP131 DENSITY ALT 700FT
CYUL 051812Z 22008KT 15SM -SHRA BKN045TCU BKN075 OVC240 23/16 A2991 RMK TCU5AC2CI1 SLP129 DENSITY ALT 1100FT
CYUL 051800Z 19010KT 15SM VCSH BKN045TCU BKN080 OVC240 24/16 A2991 RMK TCU5AC1CI2 SHRA W-N SLP130 DENSITY ALT 1100FT
CYUL 051700Z 22013KT 15SM SCT035TCU BKN110 OVC240 24/16 A2994 RMK TCU4AC1CI3 SLP139 DENSITY ALT 1100FT
CYUL 051600Z 20011G16KT 15SM SCT035 BKN110 BKN210 23/16 A2996 RMK CU4AC1CI2 SLP145 DENSITY ALT 1000FT
CYUL 051500Z 20010KT 15SM SCT030 BKN120 BKN240 22/15 A2998 RMK CU4AC2CI1 SLP151 DENSITY ALT 800FT
CYUL 051400Z 19007KT 15SM FEW035 SCT050 BKN075 20/15 A2999 RMK SC1SC3ACC2 SLP155 DENSITY ALT 600FT
Aircraft Registration Data
Aircraft registration data reproduced and distributed with the permission of the Government of Canada.
Incident Facts
Date of incident
Jun 5, 2015
Classification
Incident
Airline
Westjet Airlines
Flight number
WS-588
Departure
Toronto, Canada
Destination
Montreal, Canada
Aircraft Registration
C-GWCT
Aircraft Type
Boeing 737-600
ICAO Type Designator
B736
Airport ICAO Code
CYUL
This article is published under license from Avherald.com. © of text by Avherald.com.
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