London City cancels flights as freezing fog returns — here’s why it’s a problem for pilots
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Freezing fog returned to play havoc with schedules at London City Airport this morning as temperatures in the capital cooled to as low as -4C overnight.
As a result, 23 departing and arriving flights were cancelled at the hub in the heart of the capital, while others were forced to divert to London Gatwick to ensure safe landings.
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Cancelled inbound flights included those from Edinburgh, Amsterdam, Berlin, Dublin, Dusseldorf and Rotterdam.
One of two diverted flights from Edinburgh — BA8713 — was forced to hold over the Sussex countryside for half an hour (after earlier circling briefly over Coventry) before landing at Gatwick two and a half hours late.
The issue appears not to have had the effect it had on Heathrow last month, when BA axed more than 150 flights to and from the airport amid a cloak of freezing fog.
Why London City?
Part of the problem for London City is its sea-level position and proximity to the River Thames, which make it prone to foggy conditions.
On top of that, the airport is classed as “Category 3” — meaning pilots receive special training to land there.
Because of its geography, nestled among the skyscrapers of London’s Docklands, London City has an unusually steep approach angle (known to pilots as “glide scope”) of 5.5 degrees, according to FlightGlobal.
That is almost double the three-degree average approach angle of Britain’s other airports, making it much more difficult to land.
Add dense fog to the equation, and you’ve got a recipe for a very tricky landing indeed.
What is freezing fog?
To understand what freezing fog is, it helps to first understand how fog forms.
Fog and mist are, in effect, just clouds sitting on the ground. Mist is defined when the visibility is greater than 1 kilometre and fog is when the visibility is less than 1 kilometre. There are several different types of fog, each caused by different weather conditions.
Just like clouds, fog is formed when invisible warm water vapour rises from the earth’s surface and condenses into visible water droplets as the air cools. This is known as the dew point.
Freezing fog essentially forms in the same way clouds do when the land cools overnight under clear skies. It just happens much quicker than clouds, hence why it hangs at ground level.
According to the Met Office, as heat radiates back into space under clear skies, the earth’s surface cools, reducing the air’s ability to hold moisture. That’s when water vapour condenses into droplets.
However, when fog forms in temperatures that are below freezing, those tiny droplets in the air do not turn to ice because liquid needs a surface to freeze upon. Instead, they remain in a “supercooled” liquid state, essentially floating about in search of a surface to freeze on.
Why is freezing a problem for planes?
When an aircraft comes into contact with freezing fog, it will freeze on every surface exposed to the cold air. Most parts of the plane can withstand this as the cabin’s air conditioning and warm fuel in the wing tanks are quite often enough to melt any ice created by freezing fog.
However, there is one part of the plane where freezing fog can become an issue: the engine’s fan blades.
The rapid spinning of the front fan of an engine causes the air pressure around it to drop and, with it, the temperature.
The longer the engine turns in these conditions, the more ice can build up along the fan blades — an issue most common when the engines turn at low speeds during taxiing, especially after landing.
Not only does this ice add extra weight to the engine but if it breaks off unevenly, it could cause a weight imbalance and lead to vibration and stalled airflow through the engine.
When a plane lands and ice is found on the fan blades of its engine, it must be de-iced by maintenance engineers. this can take time, and inevitably cause delays.
Another obvious reason that fog can be dangerous is that it reduces visibility.
Can I get compensation for delays caused by freezing fog?
The only way airlines can defend a claim for compensation is by proving that there were extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken.
Unfortunately, in this case, an airline would most likely refuse a compensation claim on the grounds that freezing fog is an extraordinary circumstance.
However, like snow, this could be grounds to argue otherwise.
“It is not exceptional to see snow in December,” said flight compensation expert Coby Benson of Bott and Co. Solicitors. “In fact, there was a Court of Appeal ruling where the judge ruled that extraordinary circumstances, including bad weather, has to be, in his words, ‘freakish’ or ‘wholly exceptional’. So you have to ask yourself, is snowfall in December really freakish or wholly exceptional? No, it is clearly not.”
He says he has won many cases against airlines refusing compensation where the delay or cancellation was caused by snow. “Airlines should be prepared to face disruption in snowy weather,” he added.
By that token, is freezing fog during winter — especially at an airport prone to fog — “freakish” or “wholly exceptional”? It is a question worth asking if your flight was cancelled by fog.
There is, however, a “big caveat”, Benson points out. If a disruption is caused by an Air Traffic Management Decision (ATMD), an airline can fall back on that as a reason not to pay out. “There are many ATMDs made during the day,” he says, “and I would expect fog to be an extraordinary circumstance because it’ll affect air traffic control.”
To best understand how aircraft fly in low visibility, it’s best to hear from a pilot themself. So here is The Points Guy’s resident commercial aviator Charlie Page to explain.
Low Visibility Procedures
When the visibility at an airport drops below a certain level, normally 600 metres, the airfield switches to Low Visibility Procedures — or LVPs. Under LVPs, the way in which ATC and aircraft operate changes significantly.
One of the main changes during LVPs is the holding points around the runways. In good weather conditions, aircraft waiting to enter the runway hold at the Category 1 (CAT 1) holding points. However, when landing in fog, the integrity of the signal that the runway sends up to the aircraft (more on this later) is vital. As a result, aircraft must wait farther back from the runway, at the CAT 3 holding point.
In addition to this, each aircraft needs to be given a longer ‘runway occupancy’ time. Once a departing aircraft has crossed the CAT 3 holding point, it is deemed to be on the runway until it is airborne. The same with landing aircraft – they are not clear of the runway till passing the CAT 3 holding point.
Some of these CAT 3 holds can be hundreds of metres from the normal CAT 1 holds, so the runway occupancy time is massively increased. This is a large part of what causes delays.
Taxiing the aircraft around the airfield in thick fog is actually more difficult than the actual takeoff or landing. Even at your home base, familiar taxiways suddenly become alien and you can quickly become disorientated. Paying attention to your surroundings is vital. Inadvertently taxiing onto a runway could prove catastrophic.
In order to prevent getting lost, we conduct a departure briefing before we push back from the gate. This will include the planned taxi route to the runway. It is a great time to highlight any areas where we might take a wrong turn and end up somewhere we don’t want to be.
In order to assist us as we move around the airfield, we have a couple of maps. One on our iPads and also on one of the flight deck screens. Using both of these together, the PM (Pilot Monitoring) is able to assist the PF (Pilot Flying) in which turn to take and when. If there is any confusion as to where we are going, we’ll bring the aircraft to a full stop and let ATC know. It’s much better to ask for help than to taxi onto an active runway.
Once we’ve found our way to the CAT 3 holding point, it’s time to start thinking about the takeoff. Depending on the facilities available, each airport publishes the minimum visibility required to depart. That said, the visibility at the start of the runway could be quite different to that half way down, some 2 kilometres away.
In order to provide pilots with the best possible information, sensors are positioned at the start, middle and end of the runway — known as the “touchdown”, “midpoint” and “stop end”. These sensors measure the visibility in metres and are reported to the pilots. For example “RVRs are 130/350/450”. It’s these figures that determine the takeoff minima.
That said, the minima published by the airfield is the absolute minimum allowed. Airlines themselves will have their own minima down to which they will allow their aircraft to takeoff. This is normally 125 metres. Contrary to this rule, the Boeing 787 Dreamliner, which I fly, is different. Because of the Head Up Display (HUD), we are able to reduce the takeoff minima down to just 75 metres.
However, as I mentioned previously, the RVRs may be different at each stage of the runway. On takeoff, we require all three reported RVRs to be at least the minima required. So, on a 787 I would need RVRs of 75/75/75. If any of the three values was below 75 metres, I would not be allowed to takeoff.
Once we as a crew are happy with the reported RVRs, it’s time to go. As we start to pick up speed, the PF (normally the Captain in LVP takeoffs) is staring straight down the runway, keeping the aircraft on the centre line with their feet. The PM is scanning the engine and aircraft indications like a hawk, ready to call out any problems should anything occur.
Hurtling down a strip of concrete at 180 mph when you can only see 75 metres in front of you is a pretty disconcerting experience. You have to keep faith in ATC’s ability and the procedures that all aircraft follow to keep the runway ahead of you clear. It’s exactly why, when taxiing, that you don’t want to end up on a runway by mistake.
Once airborne, the layer of fog may only be a couple of hundred feet thick. As you burst through into clear blue skies, it’s almost as if entering another world.
Whilst there is always plenty of time to prepare for a takeoff in fog, the sudden formation of fog on a runway may catch out some poorly prepared crew. During the flight, a good crew will always be monitoring the weather at the destination. If the temperature continues to drop but the dew point remains the same, it’s an indication that fog could be on its way.
If LVPs are in force, this will be notified to the crew via the airfield’s information broadcast. This can either be picked up over the radio, or via Datalink for aircraft equipped with this technology. In addition to the usual weather, it will also give the latest reported RVRs. However, these should always be treated with a pinch of salt, as RVRs can change minute by minute.
In order for the pilots to guide the aircraft towards the runway, we must hook onto a signal that is projected up from the runway, the Instrument Landing System, or ILS. The ILS actually comprises of two signals.
The first, the localiser, sits short of the runway threshold but in line with the middle. This shows us where we are in relation to the centre line of the runway. The second beam projects upwards at a 3 degree angle, from a point roughly 1,000 feet into the runway, abeam the touchdown point.
The aircraft detects these signals and displays them on the screens in the flight deck. We then instruct the autopilot to latch onto these signals and follow them to the runway.
Like on take off, each runway, and specifically the approach to the runway has minimum weather criteria governing when we can and cannot make an approach. ILS approaches are used in foggy conditions as they have the most accurate signal.
The minima dictates the minimum RVR needed to commence an approach to land, the start of the approach normally being 1,000 feet above the ground. Once again, the three different RVRs will be reported to the pilots. This is where it gets a little complicated.
For an ILS approach to a runway, there are various different minima, depending on the equipment the aircraft has on board. For now, I’ll just discuss the most useful type of approach, “CAT 3B with no decision height”.
On these kind of approaches, we can land with a visibility of 75 metres. Also, there is no requirement to see anything out of the windscreen before allowing the aircraft to touch down, such is the accuracy of the systems. There are even redundancies built into the systems so that in the case of certain failures, the aircraft can still continue to touchdown safely.
The landing itself is what’s called an auto-land. The autopilot remains engaged all the way to touchdown and whilst the aircraft is decelerating on the runway. By leaving the autopilot to do the physical flying, both pilots are able to give all their attention to making sure the aircraft is doing exactly what they want it to be doing. If the slightest thing starts to go wrong, they will notice immediately and take the appropriate action.
As the aircraft passes over the threshold of the runway, the autopilot raises the nose slightly to slow down the rate of descent. Quite often, auto-lands are fairly agricultural. The autopilot is great at putting the aircraft in the right spot, it just sometimes lacks some grace. With the wheels firmly on the ground, we can apply the reverse thrust and allow the braking system to slow the aircraft down. All this time, the autopilot is still keeping the nose tracking down the centreline of the runway. When we have reached taxiing speed and have identified a runway exit, the autopilot is disengaged and the toughest part of the arrival begins — the taxi to the gate.
Flying in foggy conditions provides challenges to pilots that are only experienced a few times a year.
That said, it isn’t always easy to predict when the conditions will fog out, so pilots have to be prepared at all times. Taxiing to and from the gate is potentially the most dangerous stage of the flight, with the reduced visibility making it difficult to discern the aircraft’s surroundings.
That said, pilots train regularly in the simulator for operations in foggy conditions so when it happens for real, they’re well practiced in what to do.
But that doesn’t mean that, sometimes, visibility is too low to land safely, so a flight must be diverted to another airport.
As for freezing fog, it adds an extra layer of complexity to flight safety if ice forms on an aircraft’s fan blades. Then they need to be de-iced which can cause delays.
Additional reporting by Matt Blake.
Featured photo by Charlie Page/The Points Guy.
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