DNMM/Lagos has seen 2 runway excursions in 3 months. This is high, so why?
I am not actually going to answer that
I am not an accident investigator and I don’t of course have any more information than the next person. But I do think it might be a good opportunity to talk about runway excursions in general, and also take a look at any particular hazards or challenges that Lagos airport might have which could be factors in potential excursions.
What is a runway excursion?
It is what it sounds like – an airplane excurting off the runway surface and onto whatever comes after that – grass, mud, water… Longitudinal excursions are not the only ones though, lateral excursions are as likely to occur as well (the ones that go off the side of a runway).
With the exception of serious technical issues impacting the stuff that slows you down or the stuff that controls your direction, runway excursions should generally be an avoidable event. Yet, they are the third main cause of fatal accidents (accounting for 17%) and the number one cause of hull loss accidents (a whopping 36%).
So understanding the factors which can contribute to these is important because then, hopefully, we can mitigate them a little more.
What can be a factor in them?
Longitudinal Excursions: ‘Off the end’ of the runway
There are a few factors which do often seem to pop up in the reports reviewing runway excursions. Things like:
- Wet or contaminated runways
- Other nasty weather conditions
- Unstable approaches / poor energy management
- Long landings / high speed on landing
- A combination of factors…
Tailwinds should be taken into account in our performance checks, however, wind is a fickle thing and is rarely steady. This is why our performance considers 150% of the tailwind component – so we have accounted for extra wind and extra potential detriment to our stopping performance.
Aircraft also tend to have a tailwind limit – 10, 15 knots sort of a thing. So, while tailwind is a major threat, it is something we should be accounting for in our performance and already considering in our briefings. Crew should always take the wind given by the tower, but if the wind on approach is significantly different, don’t push it!
The same goes for wet and contaminated runways. This is a tough one because we are reliant on the details of the runway condition being correct (Lagos has been reported for not updating their ATIS regularly). A recent change to a new ‘Global Reporting Format’ is designed to provide pilots with condition information and more accurate braking and control performance info.
Again, there are limits to what an aircraft can (and should) land in, and there are techniques for landing in wet conditions to help prevent aquaplaning which leads to a loss of deceleration and skidding of the aircraft.
Nasty weather is a common factor, and it is not as simple as simply not landing – areas which experience severe or significant weather tend to experience it over a wide area leaving pilots in a position where they can’t necessarily route to an optimal airport, so much as land at the least bad one. Good planning, fuel monitoring and weather awareness is a key part of this, but it doesn’t mitigate every situation.
Unstable approaches tend to mean fast or high. Either way, they mean a potential touchdown at a higher speed, or point on the runway which then negates much of the landing performance which relies on an expected dissipation of energy, and touchdown in correct place. Ensuring the approach is stable is critical and if it isn’t, trying to ‘fix it’ at touchdown probably won’t end well. Go-around, try again!
Long landings beyond the touchdown zone are another ruiner of your nicely done performance calculations. There are situations where touching down outside the TDZ might be the safer option than a baulked landing, but making this decision at 5 feet probably isn’t the place to do it. A long landing especially if carrying speed, can mean a softer touchdown and higher risk of aquaplaning.
In most if not all cases, a combination of factors are probably present though, including technical issues with deceleration devices. Understanding aircraft limitations and the impact of technical issues is important because on their own they might not appear to matter, but when combined it can be a recipe for, well, skidding off the end of the runway.
Lateral Excursions: “Off the side” of the runway
A main factor in this is probably the good old crosswind. I’m no trainer and am not ashamed to say I took a while to really get the hang of these (I still need practice if I am being honest!) There is a technique, it will vary from aircraft to aircraft, as will the limits to the strength of the crosswind you can land in.
Take the mighty A380 – it has a 40 knot crosswind limit. Pretty big! The airline I operated for also had an FO limit of 20 knots. Now, it does land well in them because it has a big old rudder, but this rudder can also cause issues if an incorrect amount is applied, or if the pilots gets into a sort of wobbling chasing it situation (google A380 into EDDL to see).
Aircraft with long wings, low slung engines etc also tend to have limits to the amount of bank which can be applied on touchdown – so if the aircraft is not aligned with the centreline, there is not much you can do about it last minute, and if there is a particularly large lateral deviation, this can bring an aircraft close to the edge of the runway quite quickly. Add in a gust, a failure, contamination…
Lateral excursions on take off are also a concern to consider. With minimum runway occupancy time pressures, separation etc, there can be pressure on the lining up aircraft to commence the roll before being laterally aligned. Applying takeoff thrust while at an angle is going to send the aircraft zooming forward at that angle making it harder to bring the aircraft straight. One of the worst times to have a failure of the critical engine is at low speed when the rudder is not yet fully active.
There have also been some events in the past of aircraft inadvertently lining up with the edge of the runway for the takeoff roll – particularly at night if lighting is limited, or during low vis conditions.
What do we have to help?
Well, the usual – practicing in the sim, following SOPs, good monitoring, and sticking to limitations. But some aircraft have a little more as well.
ROW/ROP is an Airbus design which yells at the pilot if the built in checks decide the runway is too short for the conditions. It looks at things like weight, ground speed, OAT, configuration and CG, airspeed and wind, and will provide the pilot with a caution ‘If wet: rwy too short’ and a warning ‘Rwy too short’. The ROW is the overrun warning, the ROP is the protection which comes into play once on the ground. This looks at the deceleration rate and will command max reverse if the end of the runway is imminent.
Interested? Go read about it here.
Other manufacturers offer similar systems that warn if an approach is unstable (by monitoring speed and ROD), or which give runway distance remaining call outs.
On the ground, airports are now installing more and more EMAS systems – Engineered material arresting system. These help slow an aircraft down quickly if it has overrun, and are made from fire resistant material to minimise further threats. There are 121 runways at 71 major airports in the USA with these installed.
Check out the FAA page on it here for more info.
Crew awareness of runway length remaining is key when taking off or landing in gusty, windshear conditions. Windshear on takeoff is a tricky one because it might ‘invalidate’ your V1. Likewise, experiencing a brake failure on landing might be better spotted if you have an idea of deceleration distance. So, knowledge of runway lights for determining runway length remaining is important.
Shorter runways (EGLC/London City is a good example) often have light embedded in the runway to show the point by which the main gear must be down. Otherwise, the TDZ marks are a good giveaway…
If you’re not bored already then let’s look at Lagos.
DNMM/Lagos has a 3900m runway and a shorter 2745m runway (shorter but still a decent length). Runway length not an issue: Check.
Runways 18L and 18R both have ILS approaches to them, and standard 3°ones at that. Precision approaches: Check.
AHA! Runway 36L/R do not have published approaches. Instead, they both have a circle to land. These are far more challenging, and also mean runways 18L/R are probably favoured in a tailwind.
Lagos also sees some pretty challenging weather – it gets hot and stormy, and storms mean downpours which can quickly lead to wet runways and potential aquaplaning.
The reason for the two recent excursions is not yet known, but it will be a good learning opportunity once the reports are released.
Runway Excursions are a issue!
They are a BIG issue but the good news is there is a lot crew can do to prevent them. It just takes awareness, knowledge and sharing of experience.