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Korean Air Flight 801 crashed during bad visibility conditions in Guam because the ILS was out of commission and the pilot thought it wasn't.

My question though is why he even tried to land there in the first place. Wouldn't an airport be closed during bad visibility conditions in that scenario? Or is it possible to land without ILS in bad weather conditions?

I seem to remember someone talking about a step-chart or something like that as an alternative to ILS, is that what would be used in a case like that?

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"Why he even tried to land there in the first place" - isn't that obvious? He thought he ILS was working. – Jon Story Mar 30 at 15:27

The answer is "Maybe".

An ILS is actually comprised of two signals - a localizer (which lines you up with the runway) and a glideslope (which guides your vertical descent).
If the glideslope is out of service (as it was when KAL801 was flying its approach) you fly a localizer-only approach, which requires visual contact with the runway at a higher altitude.
In this case as shown on the plate below the ILS can be flown down to 531 feet (in red); the Localizer-Only approach can only be flown down to 1440 feet (in green).

Approach Plate

So as long as the crew had the minimum visibility and cloud height requirements (and observed the altitude restrictions on the approach) they could legally fly into the airport on a localizer-only approach

In the case of KAL801 there were a few contributing factors to the accident (go ahead and skip to page 142, unless you really like reading NTSB reports):

  1. The glideslope may have been receiving false signals.
    The crew should have disregarded the glideslope indication anyway as it was NOTAM'd out of service, but it's possible the crew followed the needle anyway.

  2. The DME fix for the approach is not located at the airport.
    This may have caused the crew to believe they were closer to the airport than they actually were.

  3. Because of (1), (2), or some other reason, the crew flew below the Localizer-Only minimum altitude.
    This is ultimately why they hit the hill -- they were too low during the approach.
    Had they been at or above the localizer-only minimum altitude they would have cleared the obstacles.

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Note that this approach was changed post-crash -- it used to use the UMZ DME, but that's what likely confused the KAL crew. Now, there's a DME colocated with I-GUM that yields a more sensible set of readings (i.e. ones that don't go to 0 and back up again as you head towards the runway). Also, if you make sure you cross the BOLFY fix (aka 5.1 DME or right over the UMZ VOR) at or above 1440, you can go down to 980' ASL without the glideslope or being visual to the runway. – UnrecognizedFallingObject Mar 4 at 12:35

Airports don't typically close because of visibility. In fact, ATC won't stop a pilot from flying an approach just because the weather is below minimums. There are a couple of reasons for this:

  1. The determining factor for landing is flight visibility, which may be different from the ground visibility that ATC has access to. Under US Part 91 rules, a pilot may fly an approach even when the reported weather is below minimums, and if they have the required flight visibility and are in a position to land, they are allowed to land. Part 121/135 operators aren't even allowed to try the approach though. ATC does not have to query the pilots to see which rules apply to them. They assume that they won't ask for/accept a clearance if they aren't allowed to.

  2. Different aircraft have different equipment and different pilots/operators have different training and rules. For instance, one operator may be approved to use a Heads Up Display to land when the visibility is below landing minimums for another operator that does not have one. ATC assumes that the pilot knows what they are doing and will only do what is legal for them.

As far as this specific case, @JayCarr has done a good job of answering what heppened here. The pilots simply flew the approach incorrectly....

In regards to the title of your question (which is a little bit different from the body of your question), each approach has the minimum flight visibility listed for it at the bottom of the approach plate. Unless specific authorization is received to reduce the visibility (as in the HUD example above), no it is not legal to land if the visibility is below minimums.

Each approach procedure also lists minimum altitudes that you may fly to at various points during the approach. If an ILS is out of service, there is very often another type of approach that may be flown that usually prevents the pilot from descending as far.

A very common type of approach plate will have an ILS approach combined with a Localizer approach. Since an ILS is made up of a localizer for horizontal guidance and a glideslope for vertical guidance, if the glideslope becomes inoperative then the localizer can still be used on its own. A localizer only approach without vertical guidance, uses "step-down fixes" to indicate when descents are allowed. Basically, you determine when one of these fixes is crossed and you are now allowed to descend to the next minimum altitude. From the side it looks kind of like a staircase with the last "step" being flown visually in order to land.

An example of the bottom portion of an ILS or LOC approach plate:

Approach Plate Minimums

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I'll add to this where CatII and CatIII approaches exists ATC doesnt clear you specifically for those variants. They only trust that you are legal to land in the current conditions. – casey Mar 15 '14 at 20:40
This is also a much better example of the step-down chart on an approach plate (since it has a bunch of steps on it :) – voretaq7 Mar 15 '14 at 23:09
@voretaq7 That's why I picked this chart. I've flown this approach a few times and entry time I just shake my head! – Lnafziger Mar 15 '14 at 23:24

In the case of Flight 801 the pilot was using a DME system. This is basically a series of transponders that tell you how far away they are located, and you figure your position based on how far you are from the DME and how that correlates to their position on your charts. You also have a step chart that tells you how far from the runway they are and what altitude you should be at when you get to the DMEs.

What happened with flight 801 is that the pilots were not aware that the Guam airport had it's final DME about 3 miles short off the threshold, instead of right at the end of the runway. They thought the last DME was just about where they were supposed to land (faulty training, it would seem). When they got close to the ground and finally broke the clouds they realized their mistake and tried to do a go around, but the engines couldn't spool up in time... They basically just landed 3 miles short of the runway.

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