No decision has been taken yet for a metro route to airport. As per latest reports, BMRC is keen on the Thannisandra-Bagalur route (past Nagawara) since it saves on distance and since there is pressure to include airport connectivity by metro rail as part of phase-2 itself. GoK on the other hand is keen on a route along Ballari road as they have notified land adjacent Ballari road for it earlier (when hi-speed rail was thought of as an option).
A more careful analysis confirms that the route along Ballari road in continuation of the Carmelaram-Kogilu /Yelahanka line (and not the Nagawara line) is the best suited despite some increase in distance - refer to the map below that shows planned routes for all three phases.
It goes without saying that a non-stop or limited stop line to airport will become a huge financial burden for BMRC as ridership will never be sufficient (it will be an echo of the case of Delhi hi-speed rail, for sure).
If the Gottigere-Nagawara line is extended to KIAL via Hebbal along Ballari road, it will throw the route plans for phase-3 out of sync with efforts made so far. If a direct line past Nagawara is planned, it will have viability issues as areas past Nagawara are yet to develop substantially, particularly areas past Kogilu cross along the route (east of Ballari road) that are somewhat under developed. Building or planning many stations en-route may hence not yield returns for a long time.
Thus, it becomes obvious that Yeshwantapur-Carmelaram line (in phase-3) is the one best suited with many benefits in comparison to the Gottigere-Nagawara-KIAL direct line via Bagalur (despite being longer).
(3) Airport line along Ballari road can be taken on the surface past Bagalur road cross. Even with six or eight road underpasses and about a dozen pedestrian skywalks over a surface line, it will still result in substantial savings by avoiding elevated viaduct for about 13.5km (Bagalur cross till KIAL).
(4) Surface stations can be added in between later much more easily with some land acquisition, On an elevated route like the direct Nagawara-KIAL route, stations will have to be built along with the viaduct, or at least the necessary heavy piers positioned during viaduct construction itself which is a huge disadvatage as it locks up huge funds with no uncertainty about their use.
(5) The Carmelaram-Yelahanka line is much more centrally located in east-west terms and is therefore more easily accessible from eastern as well as western parts of the city.
(6) Extending Gottigere-Nagawara line towards Hebbal along ORR and then to airport along Ballari road would mean termination of the ORR-east line at Nagawara instead of Hebbal, which is not desirable.
(7) There will be no conflicts in existing route plans for all lines.
(8) Offers possibility for slower extension of the Gottigere-Nagawara line past Nagawara, in step with development.
A separate airport coach (with more seating and luggage space) can be attached to some metro trains for airport passengers and these trains can run at reduced frequency. All other trains at higher frequency can terminate at Kogilu. Security luggage checks at few select stations on the line can be arranged (example: Dairy circle in south, Sir Vishveswaraya in CBD, Hebbal and Yelahanka).