What changes do narrow-body airliners undergo to serve (relatively) long routes?

Question

I will try to pose a well-structured question here on the longest routes ever or currently served by small, narrow-body airliners - specifically, the Boeing 737 and the Airbus A319 and A320.

Set aside special versions in the likes of the Boeing 737-700BBJ, what makes a small, narrow-body airliner serve routes that are longer than those on which it is usually deployed?

Which is to say, what is added or changed, on the design side of things, to the structure or operability of planes that are not specifically designed for long distances, to make them capable of flying longer distances? I guess the answer is ETOPS, but I am not sure of what this implies from the operational standpoint.

For my moderate understanding it is quite impressive that narrow-body birds can fly from, say, Tokyo Narita to Mumbai - a distance of approximately 6,800 km or 4,220 mi (All Nippon Airways Boeing 737-700ER, flight NH829). Another example is Sunwing Airlines flight WG481 from Vancouver to Punta Cana (a Boeing 737-800 which flew for 5,930 km or 3,690 mi).

A list of such unusual performances can be seen here.

Answer 1

The 737 ER (Extended Range) differs from the rest of its family by having an extra fuel tank extending its effective range.

Legally speaking (for US registered aircraft) you don't need to carry life preservers for each passenger, if you are only flying over land routes. So if a plane is being used for say a JFK $\rightarrow$ Nashville route, they would not be required to carry life preservers; but if they decided to use the plane on an over water route, they would have to outfit it as such.

Inter-Country travel may also effect the outfitting of a plane, as various countries have different equipment requirements (although most are the same now). Long haul routes may very well translate to non domestic routes, which means the plane may need to be outfitted with extra equipment that is capable of working on foreign systems.

There may be some paperwork involved around certifications as well. For example the FCC does not require domestically operated aircraft (commercial and GA) to have radio station licenses; however, it's often required by many foreign entities. A domestic only plane may need to obtain one of these licenses before heading to is destination.

On a bit of a side note the pilot must also be familiar with the destinations' regulations and methods of operation. For example here is a nice briefing from the FAA on flying into China.

Answer 2

Why long routes by narrow bodies?

Airlines use narrow bodies for medium-long range routes mainly for 3 reasons.

1. Demand
2. Market strategy
3. Fleet structure

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Demand

Some airlines choose to service medium-long range route with narrow bodies simply because they don't have enough customers for bigger planes---like the 2 flights you have mentioned, both of them are cancelled because of insufficient demand.

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Market strategy

On the other hand, some airlines are likely to provide more frequent services even with smaller planes. Business passengers are the main sources of income on those flights, they will pay extra for better a time that better suits them. That is common in flights between main cities between the east coast and west coast of the US. American Airlines uses the 3-class A321 to service 10+ daily flights between JFK and LAX (2173 nmi).

In addition, some airlines provide business-class exclusive services on the selected routes. Due to its relatively small demand, the airline will choose narrow bodies even for medium long route. BA Clubworld between London city and JFK is such an example, they service the 3000 nmi route by A318 with 32 full flat bed seats.

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Fleet structure

Some airlines only operate narrow bodies, regardless of whether they are low cost carriers. So they service medium-long route only by narrow bodies, like Alaska 870 (Anchorage-Honolulu, 2418 nmi).

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How long are routes by narrow bodies?

Both A320 and B737 series have "standard" 3000+ nmi maximum range when fully loaded, which can cover most of the "long" route. More accurately their MOTW starts reducing when range is over 2000 nmi. (Except B737-700ER)

It is hard to describe the payload/range in details. I would suggest you to have a look of it. (P.3-2-1 on A318, A319, A320, A321 & P.85-98 on B737 series)

ETOPS stands for Extended-range Twin-engine Operational Performance Standards, a rule which permits twin engine aircraft to fly routes which, at some point, are more than 60 minutes flying time away from the nearest airport suitable for emergency landing.

ETOPS is not a must when flying "long" routes, but it is necessary when flying over oceans or close to the Pole. Due to the limitation of the range, only a few routes require ETOPS, including a few transatlantic route (UK/Iceland - US East Coast), US West coast to Hawaii, as well as East Asia to Micronesia/Melanesia.