EMAS and Runway End Safety Area (RESA)
The role of the RESA and the use of EMAS has been raised by several people here – first by SGC. Thanks to the excellent diagrams/photos posted by Kulverstukas, I was able to look at what effect an EMAS would have had at Vnukovo Airport.
What an EMAS really does
Firstly let me clarify what an EMAS really does because there are sometimes misconceptions about what it can do. It is not a physical arrestor like the aircraft carrier arrestor cable, nor is it a barrier net or barricade like the military use.
It can be simplistically thought that an EMAS effectively adds 525 feet of runway with guaranteed maximum braking which is 160 metres.
Effectively because the deceleration in an EMAS varies due to several factors which can be correlated to aircraft weight – heavier aircraft do not slow down as quickly. EMAS design adjusts for this by providing longer EMAS beds for heavier aircraft to ensure it still effectively adds 525 feet of maximum braking.
guaranteed maximum braking because the EMAS does the braking for you. Regardless of brakes, tyres, systems, pilots, runway surface, or weather, EMAS gives you that guaranteed maximum braking. Staying with my simplistic 525 feet analogy, it gives a deceleration of 13 ft/s2 (which is 0.41g or a typical Boeing "Autobrake Setting MAX). The actual aircraft deceleration in the EMAS bed varies with weight and other factors from 0.38 g to 0.78 g for aircraft from the 747 to the CRJ-200.
Simplistic because there are many more factors at work, but there is a consistency in the results which suggests that simple works here.
Design EMAS for Vnukovo Airport
A standard EMAS design will stop the design aircraft at a runway exit speed of 70 knots; it has a paved 75-foot set-back, assumes no reverse thrust, and a 0.25 braking friction coefficient.
Assuming that the EMAS for Vnukovo Airport was designed with the Tu-204 as the design aircraft. Assume that the Tu-204-200 design aircraft has a MTOW of 244,115 lbs (110,750 kg). The designed EMAS structure would typically be a bed length of 375 feet (115 m) and a paved setback of 75 feet (23m); a total of 450 feet (137m) from the runway end which would easily be fitted at that end of the runway at Vnukovo.
If the aircraft left the runway at 70 knots, then it would be stopped at the end of this EMAS bed.
Performance of EMAS in actual accident
The performance of the EMAS in the actual accident would be very much affected by the speed at which the aircraft was travelling when it left the runway. The standard EMAS design assumes a runway exit speed of 70 knots, but for Vnukovo, Lyman has given us some reasoned estimation that the speed was 100 knots.
At 100 knots exit speed, a MTOW Tu-204 would leave the paved 75 feet area and enter the bed at 165.17 ft/sec (98 knots). It would leave the EMAS bed still travelling at a high 71 knots. From there it is about 123m to the first significant drop-off. Assuming very poor braking on the snowy ground, it would be travelling at 64 knots or more when it reached the first significant drop-off. The actual aircraft was light, so its deceleration in the EMAS bed would be slightly higher but the effect on speed would only be 5-6 knots lower. All this assumes no forward thrust, which would only have made matters worse.
So would the EMAS have saved the day or even mitigated the accident? Unfortunately in this case it appears not.
Last edited by OverRun; 3rd Jan 2013 at 04:04.
Reason: Add light op weight effect