14 May, 2003: If god had meant us to fly...

[ Home page | Web log ]

The BBC's `mockumentary', The Day Britain Stopped has been attracting quite a bit of attention. The basic notion of the programme is that A Bunch Of Bad Things Happen To The British Transport Network All At The Same Time. The result is chaos of a uniquely televisual kind, with the added bonus that most of the programme could be stuck together out of archival news footage.

It turns out that the BBC have put the programme on the web, in crappy RealPlayer format. So I thought I'd watch it. I'll spare you any comments on the quality of video-over-the-internet, except to remark that it would be nice if video and sound were synchronised, just like they are on real TV. But that's by the by.

The basic sequence of events in Stopped is,

  1. The rail unions hold a one-day strike as a reaction to a rail crash, so
  2. there's a larger quantity of traffic on the roads, which
  3. results in accidents on the motorways, leading to
  4. gridlock all over the place and motorists trapped in their cars freezing to death, so that
  5. air-traffic controllers can't get to work, and
  6. a momentary lapse causes a mid-air collision over west London between two aeroplanes leaving Heathrow airport.

Now, everyone likes disaster fiction, and this was quite well-produced. But much of it doesn't make sense. For instance, as Nick Barlow points out,

Also, there was one fact that was wrong. After the accident, they referred to tailbacks on the M25 growing at `a mile a minute' which means that the back end of the queues was growing at 60mph. To understand how strange that would be, imagine driving down one carriageway of a motorway at 60mph when the opposite carriageway has been closed because of an accident. You'll drive past a long queue of traffic caused by the closure, but eventually you'll reach the end of the queue. For it to be growing at a mile a minute, the end of the queue would be travelling with you as you went at 60mph. I have heard before a figure of 15mph for the speed at which queues on the M25 can grow after a closure, though.

-- actually, we can do a bit better than argument by assertion. (Sorry, HTML is a crap medium for maths....) Suppose that vehicles length l drive at speed v at intervals of s. Suppose then that they reach an obstacle and stop, with cars in the queue separated by a distance d, and that the queue grows backwards at speed u. Cars join the queue at a rate f, so u = f (l + d).

Queuing problem

Clearly f = (u + v) / (vs + l), so, solving for u, we find that u = v (l + d) / (vs - d). Choosing v = 26m/s (60mph), s = 1.5s, l = 4m, d = 1m, we get u = 3.4m/s, or a bit over 7mph. Tweaking the parameters for higher input speed or a larger mean vehicle length (to take account of trucks and so forth) could yield Nick's 15mph. 60mph would require pretty extreme conditions, for instance dropping the separation to half a second and tripling the mean length of vehicles (which would correspond to traffic consisting almost entirely of heavy trucks).

The most controversial bit of the programme was the mid-air collision. The supposed sequence of events was that an aircraft coming in to land was forced to `go around' because a previous aircraft had not cleared the runway. At the same time an aircraft on the second (parallel) runway was cleared to take off. Both aircraft turn left, but second 'plane turns more rapidly and the two collide at an altitude of 1,800ft. The BBC have produced some pictures of this, of which this is the most important:

Mid-air collision scenario

(It was notable that the programme didn't show any of these diagrams, despite its `documentary' format.)

Naturally, the air-traffic-control people do their best to avoid embarrassing accidents of this type, and of course have procedures designed to prevent their occurence:

The missed approach procedure for all Runways is:

Climb straight ahead to 3000ft then as directed by ATC.

Normally missed approaches from Runways 27R/09L will be turned, after co-ordination, towards the North and from Runways 27L/09R towards the South. Missed approaches from Runway 23 will be turned after co-ordination to the left. It is stressed that if a decision is made to turn a missed approach aircraft towards the departure runway, the arrivals controller must ensure that specific authority is obtained from the departure controller and acknowledged.


Aircraft carrying out a missed approach shall not be instructed to make any turns below 1500ft QNH unless there are overriding safety reasons, in which case the SVFR Controller is to be informed immediately.

So, to start with, the scenario presented -- with the 'plane going-around beginning its turn well below 1,800ft -- would apparently require special action not explained in the programme. Secondly, departures are supposed to be cleared with the arrivals controller:

On becoming aware of, or on being informed of a 'go-around' from any runway the Air Departures controller is to:

a) Suspend departures until otherwise agreed with the appropriate Radar Director and Air Arrivals.

b) Co-ordinate with Air Departures to establish separation between the `go-around' and departing traffic. Air Departures is required to inform Air Arrivals of all departures and Air Arrivals is to acknowledge this information. Actions taken to establish separation between the missed approach aircraft and any departing aircraft, including details of any tactical radar headings that are being used must be confirmed with Air Departures and an acknowledgement obtained.

-- whereas in the programme, a 'plane was cleared for departure regardless of the go-around in progress. It's not clear how this is supposed to have worked, since permission ought to have been required from the arrivals controller, and if this was given it wasn't explained why....

Although near-misses do occur in this kind of situation -- see, for instance, this report from 1996 --- it seems that procedures have been tightened up and aeroplanes do now have collision-avoidance RADAR. In any case the altitudes and distances in the programme don't make much sense.

Another rather dubious detail concerned a character who was supposed to be on a flight from Glasgow to London. The flight was diverted from Heathrow to Gatwick, and the programme claimed that this might have resulted in the 'plane running out of fuel. This seemed a little bit surprising, but this incident report describes a 'plane diverting from Gatwick to Stansted which carried enough fuel to hold for eight minutes plus enough for its preferred diversion to Stansted. In conditions as depicted in the programme with very long hold times, an out-of-fuel emergency is probably not inconceivable. This article describes an airline policy which covers

taxi delays, ATC delays enroute, holding fuel, fuel for an approach into the destination, a missed approach, diversion to the alternate with a full approach there, then a 45-minute cushion on top of that.

... but there's no way to know how domestic carriers actually determine how much fuel to carry. So this bit was probably just about plausible.

Obviously Stopped was designed to be controversial, which explains comments from its writer such as,

The aviation industry will close ranks and cast doubt on the credibility of this.

-- or, to put it another way, ``I'm not listening''. Hardly a helpful way to start a debate.

And in other news

Copyright (c) 2003 Chris Lightfoot; available under a Creative Commons License.