Is this Acceptable? When a runway strip isn't wide enough

This week, on Sunday, 14th January 2018, a Pegasus 737-800 veered off the runway at Trabzon, Turkey and came to rest on a steep slope quite close to the Black Sea. Whether by expert skill or good fortune, no one died in this runway excursion but it would look like the aircraft is going to take a bit of work getting back into the air.

Obviously, it is way too early to speculate on the causes of the accident but as airport safety nerds, I think its okay for us to have a look at the role the runway strip played in this event. I've already seen a few internet comments question the compliance of the runway strip and I have to admit, on first viewing, I too thought the strip looked narrow. However, if Google Earth is to be believed, these comments and first thoughts might not be correct.

Runway Strip Standards

I'm not familiar with the specific aerodrome standards in force in Turkey, so let's use ICAO's Annex 14 as the basis of our discussion. Let's start with the definition of a runway strip.

Runway strip. A defined area including the runway and stopway, if provided, intended: a) to reduce the risk of damage to aircraft running off a runway; and b) to protect aircraft flying over it during take-off or landing operations.

The dual purpose of the runway strip will become important in the subsequent sections as the strip protects both aircraft on the ground and aircraft in the air.

The nature of runway strips are defined over a number of standards and recommended practices under section 3.4. For the sake of this exercise, I'm going to consider recommended practices as binding despite each state being allowed implement the standards and recommended practices as they see fit.

The width of the runway strip (section 3.4.3) for precision runways (Trabzon appears to have an ILS) must be 150 metres each side of the runway centreline* (i.e. 300 metres overall).

So, this would seem that the runway at Trabzon is not compliant with international standards.

But we need jump down a couple of sections to find some more applicable standards and recommended practices.

Starting at section 3.4.8, we find out about what I've always called "the graded portion" of the runway strip. As section 3.4.8 defines, instrument runway strips are only required to be graded 75 metres each side of the centreline and the purpose of this area is to serve aircraft during a run-off event. For precision approach runways, this graded portion is a little more complicated as outlined in Attachment A, Section 9 - which we will come back to in a moment.

Essentially, this section is creating a distinction between two portions of the runway strip in terms of their purposes. The inner graded portion is for aircraft run-off protection and the overall runway strip width is for aircraft over-fly protection.

Before we declared the runway strip at Trabzon compliant or not, let's quickly look at that Attachment A, Section 9. Under this section (9.3 specifically), a wider graded portion is required for the central portion of the runway  and this is said to be "designed using information on aircraft running off runways".

So where does that leave us with this runway and this event?

Dan's Poor Artistic Skills

Trabzon RWS Width Diagram

Using the "advanced" tools of Firefox and PowerPoint and a very basic application of perspective, I created the above diagram as a rough guide only but here are my takeaways:

  • The runway strip appears compliant at the runway end where the event occurred and along the central section, at least in the foreground of the picture above
  • The additional graded portion recommended under Attachment A, Section 9, which is said to be based on experience did not capture this particular run-off event.

It's About What Is Not Being Said

I am happy to acknowledge that the interpretation of the overall strip width and the graded portion that I have outlined above is not shared completely among airport standards aficionados. But while some of us may think airports should provide this and should look like that, not every airport location has been blessed with favourable geographic features.

My interpretation stems from the dual purposes contained in the definition of a runway strip and the specific mention of run-off in the graded portion standard. Therefore, I consider the outer portion not being there for run-off. Further, slope limitations, save one, are placed on the graded portion only and where one does apply to the outer portion, it focusses on rising slope only, suggesting that there is no maximum down slope (e.g. a cliff).

Another big contributor to my interpretation is also the historical inclusion of a "fly-over area" in Australian guidelines for aeroplane landing areas. This terminology has been transferred into the current Manual of Standards Part 139 and forms part of the runway strip standards.

A Question of Risk

Long before Safety Management Systems and risk-based systems, aerodrome standards were being set based on some form of risk assessment. The learned people that make-up ICAO's panels have been determining where lines should be drawn and they are still doing it today.

The text in Annex 14 suggests that the wider central section of the graded portion of the runway strip was introduced based on experience or, at least, some form of analysis. And we're seeing reductions in runway strip width requirements coming down the line following more recent studies into the risks associated with very large aircraft.

In the end, through this process it seems weird to say that the international standards for runway strips currently in place have deemed the picture above as acceptable. This is, of course, an oversimplification and the accident investigation may highlight factors that could not, and perhaps should not, be part of the runway strip width risk analysis process.

Waiting...

As with all accident investigations, we will have to wait for answers. And this time we have the good fortune of earning this lessons without the loss of life.

 

* there is a lovely "where practicable" in there too but I've already discussed that term in the NZ RESA Court cases here and here.

I would love to give attribution on the pictures used in this post but from where I found them, they didn't include any details. If they are your pictures, I'm happy to add attribution or any other action you may need me to take.