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Although runways 04L/22R and 09/27 at KBOS stop just short of intersecting, the blast pads for runways 04L and 09, which continue some distance beyond the ends of the runways proper, do intersect. The portions of said blast pads south and west of where they first touch are entirely (except for the area supporting the runway-27 localiser array) covered with some sort of turquoise substance (according to Google Maps, at least):

the blue(ish-green) blast pads of Boston

What is... whatever that is, and why is it there?

For comparison, here're those same blast pads back in 2018, when they had just the normal yellow chevrons:

normal blast pads of Boston

Not a dupe of this question:

  • The green-marked areas there are in entirely different types of locations compared to the ones here (those markings are along the sides of runways and taxiways; the ones here are on blast pads).
  • The markings there are the wrong shade of green.
  • If the explanation here (coloured stuff marking a no-drive/no-taxi area) were the same as there, one would expect there to be turquoise stuff in a lot more places than just that one particular set of blast pads (not to mention that, as explained in @ymb1's comment, blast pads are already no-go areas except when they're being used as stopway during a runway overrun, making extra colourant redundant).
  • The weirdly-coloured stuff in this question, unlike that in the other question, actually obliterates a set of markings that are generally highly important (the blast-pad chevrons on the affected areas), thus tending to require a stronger justification for its presence than otherwise.
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    $\begingroup$ @RalphJ: No, it doesn't, given that a) that question's green-marked areas are in entirely different types of locations, b) they're the wrong shade of green, c) if the explanation here were the same as there, one would expect there to be turquoise stuff in a lot more places than just that one particular set of blast pads, and d) the weirdly-coloured stuff in this question, unlike that in the other question, actually obliterates a set of markings that are generally highly important (the blast-pad chevrons on the affected areas), thus tending to require a stronger justification for its presence. $\endgroup$
    – Vikki
    Sep 2 at 1:50
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    $\begingroup$ @RalphJ: Yellow chevrons are already no-go unless used as stopway. I don't see the relation to the linked post. It gets more interesting as in 2018 it was normal chevrons all the way. $\endgroup$
    – ymb1
    Sep 2 at 1:52
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    $\begingroup$ Did someone do some green-screen filming there? $\endgroup$ Sep 2 at 2:30
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    $\begingroup$ Maybe it’s the last unfinished work of late artist Christo ;) $\endgroup$
    – TomMcW
    Sep 2 at 2:52
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    $\begingroup$ Have voted to leave the question open, does not seem to be a dupe. $\endgroup$
    – Koyovis
    Sep 2 at 4:50
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KBOS airport

KBOS airport, seen from East, source: Jet Photos

Short answer

In 2015, the airport owner started a project to accommodate for larger category VI aircraft (Airbus A380 and Boeing 747-8). The project consisted in Terminal E modification and runways and taxiways local pavement reinforcement.

For the project to be accepted by authorities, it had to satisfy multiple environmental criteria. The chosen solution included offsetting pavement additions by removal of an equivalent area of existing pavement. This removed portion included a part of runways 9 and 4L blast areas. Removal details:

To offset new areas of pavement, Massport proposes to remove pavement on the airfield. These areas would be planted with grass to offset grassland lost through the proposed airfield improvements.

Reinforced areas are depicted in blue below, removed areas in orange:

KBOS project: Pavement areas to be added and removed

The overlapping blast area between the two runways was ultimately decommissioned, though, as visible on Google Streetview when looking carefully, the pavement wasn't removed, and there is no grass yet.

Instead the area was painted in this green-blue color, which is the recent standard used for "NO TAXI" islands. Smaller patches of this color can be found at other places on the airport.

Runway 4L and 9 thresholds are not located at the beginning of the paved strips. This is for approach path reasons. But the unused chevron-covered portions are not part of the runway safety areas (RSA) either, in particular they are not published as stopways.

Details on markings and RSA follow.


When is green paint used on airports?

Green paint or artificial turf can be used for airport markings for areas not able to sustain the load of an aircraft. Such color is used for innermost borders of taxiway shoulders. The inner area may also be filled with green color (AC 150/5340-1 Standards for Airport Markings):

The continuous taxiway edge marking consists of dual, continuous lines (2) with each line being at least 6 inches (15 cm) in width [...] Although it is preferable for the inner portion of NO-TAXI islands to be unpaved, for example, grass covered, the inner area may be painted green or painted with striated yellow markings per paragraph 1.3.d.

An example at KBOS:

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What sort of areas can extend the runway?

Runways can be extended by:

  • A blast pad used as a protection against deterioration and projections by jet blast. A blast pad is not guaranteed to sustain aircraft weight. It is not a stopway, and it cannot be used in landing or takeoff calculation.

  • A stopway used as a last resort way to decelerate aircraft in a landing or a rejected takeoff. A stopway is able to receive aircraft without damages.

    A stopway is declared (AC 150/5300-13 §322) and its size appearing in the airport diagram and FAA A/FD can be used in calculations.

  • A EMAS is made of specific frangible materials to completely stop the aircraft with little damages to the gears. Such systems are found on 22R and 33L runway ends (visible above in white), and one is planned for runway 27 end. An EMAS is not a stopway.

  • A clearway to create a free corridor for departing aircraft.

All these areas are covered with yellow chevrons, except for clearways over water, so landing aircraft are reminded they are not part of the runway. Stopways and clearways can be included in takeoff performance computations. A clearway increases TODA and a stopway increases ASDA:

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TORA, TODA, ASDA, LDA

What are they in the case shown?

On the airport diagram (source):

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There are no size indication in the airport diagram and when looking at TODA and ASDA distances in A/FD, we see TODA and ASDA are the same than TORA:

RWY 22R:TORA–7864 TODA–7864 ASDA–7864 LDA–7046
RWY 27: TORA–7001 TODA–7001 ASDA–7001 LDA–7001

Therefore the areas are neither stopways nor clearways. They are blast pads. From FAA regulation standpoint, a blast pad is not part of the runway safety area (RSA), and its removal doesn't change the runway declared lengths.

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    $\begingroup$ So what is the green area made of? If I read your answer right it should be grass, due to the removal of the blast pad pavement to allow for the addition of pavement elsewhere on the field. Is it painted pavement or painted grass? $\endgroup$
    – randomhead
    Sep 4 at 0:33
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    $\begingroup$ @randomhead: I think the environmental requirement for removing the pavement is to maintain the same level of water draining (from what I read in the project). When you look at the picture you clearly see the chevrons behind what seems to be paint. Did they remove the pavement? I can't say for sure, but it seems the pavement is still there. $\endgroup$
    – mins
    Sep 4 at 0:50
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    $\begingroup$ @randomhead: Probably the work order said Turn [area specified] into green area, and the guy with the paint brush said "There! It's green now." $\endgroup$
    – ymb1
    Sep 4 at 1:11
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    $\begingroup$ I think I see what you mean: That whole area is still covered in pavement, but they ripped it out and covered that same area with less volume of asphalt (for better drainage), so it can't support an aircraft anymore. And then they painted it green to make it clear that it couldn't be taxied on. Am I on the right track now? $\endgroup$
    – randomhead
    Sep 4 at 2:40
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    $\begingroup$ Hm. The document clearly says that the proposed removed pavement areas (in orange in your screenshot) will be planted in grass. Apparently that didn't happen. $\endgroup$
    – randomhead
    Sep 4 at 2:50

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