I recognise that autoland and ILS can enable landings in zero visibility, which I presuppose in this question (e.g. in this video where touchdown happens around 5:22). Further, suppose that the fog is so thick that runway lights are only visible after touchdown.

  • Then how would landing be safe or reasonable?
  • Would such landings recklessly over-rely on and over-trust technology?
  • Instead, why not wait for increased visibility or divert to another airport?
  • Or does this sort of landing (video linked above) only occur as the last resort, where the aircraft must land, before depleting fuel?
  • 3
    $\begingroup$ What does "over-rely on and overtrust" mean? How is relying on ILS different from relying on your other navigational aids, or even the engine(s)? $\endgroup$
    – Dan Hulme
    Nov 13, 2014 at 10:14
  • $\begingroup$ @DanHulme: I had intended "over-rely on and overtrust" to mean 'overvalue, overplay' the reliability of technology, so excessively depend on technology. Please feel free to edit my OP; my English is imperfect. Your 2nd question: Visual confirmation is helpful and necessary, right? As a check to ILS? $\endgroup$
    – user128
    Nov 13, 2014 at 10:21
  • $\begingroup$ I think landing would be safe, but taxiing would be kind of impossible. So a landing with effective zero visibility is only useful if you don't want to taxi thereafter (e.g. if you need to land) $\endgroup$
    – orique
    Nov 13, 2014 at 10:57
  • $\begingroup$ @UpvoteLawArea51Proposal It was the "over" part that I was questioning. I'm not sure what "excessively" depending on technology is. As soon as you get into an aircraft you're depending on lots of technology. $\endgroup$
    – Dan Hulme
    Nov 13, 2014 at 11:13
  • $\begingroup$ A previous question is rather similar: aviation.stackexchange.com/q/281/3775 $\endgroup$
    – usernumber
    Feb 11, 2015 at 23:52

2 Answers 2


Such landing is safe, reasonable and completely standard business.

The ILS transmitter is rather simple, so there is not that much that can go wrong. But to qualify as category III, only which can be used with no decision height, it additionally needs monitoring circuits that check whether the correct signal is being transmitted and shut it down if it transmits incorrect signal. There is a strict limit how fast the monitoring circuit must react and the whole system is regularly tested. The monitoring system is already needed for category II, but with weaker limits for reaction time. Additionally the tower ensures no other aircraft and vehicles move in area around the transmitters where they could affect the signal propagation.

On the aircraft side for auto-land dual channel autopilot is required. So there are two systems that decode the signal and calculate the control input. If the systems disagree or if the signal fails, alarm is sounded and the pilot flying will initiate a go-around.

The controller on tower sees the aircraft on radar, so can verify that the aircraft is indeed approaching the intended runway. If the signal fails in the very last moment and there is not enough time to go around until wheels touch the ground, the aircraft won't have time to divert from course significantly, so the wheels still touch down on the paved surface. If the lateral signal fails during roll out, there is some risk of runway excursion, but that's what the safety areas are for. Runway excursions (to the sides; overruns are another matter, but they are not significant risk here) rarely result in injuries.

This is really the same level of safety as for any other system you rely on like engines or hydraulic flight controls and many other systems in the aircraft.

Note that since there is no taxi guidance, landing in zero visibility is not really possible anywhere (except, as raportech97 correctly noted below, in emergency). Some visibility is always needed (50 m should be the lowest, airport and aircraft equipment permitting) that also gives pilots chance to manually control the roll out in case of localizer failure in that phase.

  • $\begingroup$ Not possible or not practiced? If you had a fuel emergency and there was, for some reason, say, 10 m visibility, you would still land, right? $\endgroup$ Nov 16, 2014 at 2:58
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    $\begingroup$ @raptortech97: True, in emergency they may do things that would normally not be permitted. Like this Air India A320 at Jaipur that autolanded on CAT I runway way below minima (and had excursion and damaged the aircraft; there is a reason it is not normally allowed). $\endgroup$
    – Jan Hudec
    Nov 17, 2014 at 20:09
  • $\begingroup$ No DH/50m is a cat 3b limit. 150m is a takeoff limitation, both are ground/aircraft equipment limited. $\endgroup$ Nov 17, 2014 at 22:40
  • $\begingroup$ I thought Cat III required taxiway centerline lights and lead-off lights? $\endgroup$
    – rbp
    Nov 20, 2014 at 15:25
  • 2
    $\begingroup$ @rbp: I believe it does. But you have to see them to some distance to be able to steer the aircraft safely. And don't forget closest point on the ground you see from 747 is some 25 m (80 ft) away. If the furthest point you see is 50 m, you don't get to see many of the lights. $\endgroup$
    – Jan Hudec
    Nov 20, 2014 at 20:54

A completely blind landing using instrumentation is classified as a category IIIc and requires appropriate equipment both on the ground and on the aircraft. Only a handful of runways around the world support this level of landing because it is expensive and not many aircraft/pilots are rated to perform such landings. Just the lighting arrangements alone are very expensive.

Any situation in which you cannot see the full runway is very dangerous because if there is an obstruction you will not be able to see it (and vice versa). Also, how would you taxi? Having lots of aircraft trying to taxi around a low visibility airport is an invitation for an accident, because if the aircraft makes a wrong turn, and the tower cannot see the aircraft (duh) then the aircraft could move onto an active runaway. Meanwhile another aircraft taking off or landing on the runway, likewise would not be able to the obstruction to avoid it.

You can see from this description how a blind airport can quickly turn into a dangerous environment. For this reason, airports usually stop operating when visibility drops below a few hundred yards.

  • 2
    $\begingroup$ airports that have Cat III operations also have ground radar to monitor taxiing $\endgroup$
    – rbp
    Nov 20, 2014 at 15:26
  • $\begingroup$ @rpb Thanks for the info; didn't know that. Apparently using this makes taxiing EXTREMELY slow, and there is a lot of potential for error. For example, in one incident an aircraft on surface radar crossed a hold line. The captain wrote: "Confusion was caused by having more than 1 CAT III hold line for the runway, and that the hold line is printed in the wrong location on our airport diagram." $\endgroup$ Nov 20, 2014 at 15:39
  • $\begingroup$ how fast do you personally drive in fog? its a slow process. and yes there is room for error, but all human endeavors have room for error $\endgroup$
    – rbp
    Nov 20, 2014 at 20:33
  • $\begingroup$ An airport may forbid planes from taking off, and request that planes refrain from landing if possible, but I think even in conditions of zero visibility an airport would still try to accommodate a plane that is going to land one way or another, so as to minimize the danger to anyone who may be involved. $\endgroup$
    – supercat
    Aug 18, 2016 at 21:14

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