{"id":10623,"date":"2020-10-20T16:10:02","date_gmt":"2020-10-20T20:10:02","guid":{"rendered":"https:\/\/ops.group\/blog\/?p=10623"},"modified":"2020-10-21T11:40:17","modified_gmt":"2020-10-21T15:40:17","slug":"overrun-forrest-overrun","status":"publish","type":"post","link":"https:\/\/ops.group\/blog\/overrun-forrest-overrun\/","title":{"rendered":"Overrun, Forrest, Overrun!"},"content":{"rendered":"

Earlier this week the Accident and Investigation reports came out about two aircraft overruns<\/a>, on the same runway, that occurred within two hours of each other.<\/span><\/p>\n

So what was going on in UEEE\/Yakutsk back in 2018?<\/h4>\n

Or rather, what was going off, and why?<\/p>\n

A bunch of factors contributed to this double whammy of airplane excursions. First up, the runway at Yakutsk airport had been shortened for works. It was, in fact, 1,150m shorter – which is quite a significant amount.<\/span><\/p>\n

There were some Notams published about this, (and pretty decent Notams at that)<\/span><\/p>\n

A5991\/20 said –
\n<\/strong>DAILY 0000-0800: RWY 23L AVBL FOR LDG ONLY. <\/span>LDA 2248M<\/b>. TKOF FM RWY 23L CARRIED OUT BY REQ DURING THIS PERIOD. 2. DAILY 0800-2359: RWY 23L AVBL FOR TKOF\/LDG. DECLARED DIST: TORA 2248M, TODA 2398M, ASDA 2248M, LDA 2248<\/span><\/em><\/p>\n

And then there was A3621\/ 20 which said –
\n<\/strong>AD TEMPO UNAVAILABLE FOR ACFT OF FLW TYPES: IL-96-300, IL-96-400, IL-86, IL-62, A-310, A-330, TU-154, BOEING777, BOEING747, BOEING-767-400ER, MD-11F AND THEIR MODIFICATIONS.<\/span><\/em><\/p>\n

What about the airplanes, I hear you ask.\u00a0<\/span><\/p>\n

Well, the Sukhoi Superjet 100LR is not included on the list of \u201ccan\u2019t land here\u201d airplanes. However, the Notams should have at least given them pause for thought, especially since both of them had technical issues reducing their deceleration performance.<\/span><\/p>\n

Number 1 \u201cFirst to Overrun\u201d was found to have significantly worn out tires (which should have been spotted during a walk around), while Number 2 \u201cAlso Skidding Through\u201d had a thrust reverser out of action. No big deal, but factors to be considered in the context of the other conditions of the day.<\/span><\/p>\n

Talking of those conditions – the ATIS was reporting a tailwind of 6kts which is not outside anyone\u2019s limits, and of course 150% of any tailwind is taken into account for landing calculations.<\/span><\/p>\n

The braking co-efficient, however, was reported as 0.45
\n<\/b>Now, ICAO and most national authorities have moved away from reporting measured friction because they decided that, really, it is a pretty useless thing to report. There is not actually any great way to work out how <\/span>those<\/b> contaminants on <\/span>that<\/b> day will result in <\/span>whatever <\/b>friction for <\/span>whichever<\/b> aircraft – because there is no way to correlate the measurements a ground measuring device can measure in a meaningful way to what an airplane will actually experience. In other words – i<\/span>t has limited practical use in actually characterizing the runway conditions for an aircraft operation.<\/span><\/p>\n

To further add to its pointlessness, the 0.45 was not even accurate. The real coefficient measured that day was actually less than 0.3.\u00a0<\/span><\/p>\n

As slippery as an oiled-up eel
\n<\/b>Now, these pilots did do a landing performance calculation using what they thought were accurate figures. Even with their selection of only medium auto brake, and the mandatory 15% safety margin added in during in-flight performance calculations, the results looked ok and so they gave it a go.<\/span><\/p>\n

However, had they known the coefficient was only 0.3 then they would hopefully have come up with landing results similar to those calculated during the subsequent investigation. These showed that a Superjet needs about 1,598m on a dry runway, 1,838m on a wet runway and a whopping 3,650m if the coefficient of friction is 0.3. Their 15% safety margin could not even cover the extra distance because of the poor braking action.<\/span><\/p>\n

So, with one of the reversers out of action, a tailwind, an incorrectly reported friction co-efficient and only 2,248m available for stopping in, poor old airplane Number 2 never stood a chance of stopping<\/strong> in the space available.<\/span><\/p>\n