For most of the decade since 9/11, airlines invested little in passenger comfort.  Now there is light on the horizon.

Perhaps inspired by the low-cost but stylish Virgin America – the first carrier to install “mood lighting” – some airlines are beginning, once again, to invest in their product, to the tune of billions of dollars.

To understand the significance of this shift, it helps to look back.

In search of profits
Ever since 9/11, the airlines have been busy trying to recover from a tragedy that nearly devastated an industry that already was marginally profitable, at best.

The oil price spike of 2008 layered on another potentially crippling challenge to profitability.

The airlines responded by “unbundling” their fares. The idea was to offer a product so basic and stripped down that passengers would be willing to part with additional cash to buy their way out of the misery.

But the new model gave ancillary revenues a bad name. To the passenger, it meant paying for things that used to be free – checking bags, selecting seats in advance or fending off starvation with “food” that had a disturbingly long shelf life.

Change is in the air
Now the airlines are trying something different – enhancing the product.

As usual, they’ve started with the front of the plane, loading it up with seats that convert to flat beds and bringing in name-brand chefs to design meals.

Now some improvements are making their way back to economy class, where a round of cabin upgrades goes some way toward bringing the economy experience back from the Slough of Despond.

Even American Airlines, flying through Chapter 11 bankruptcy, is making significant investments to bring its in-flight experience back to its former glory.

New interior design
Several airlines are looking to a new style of cabin interior that manages to be more fuel-efficient while providing a greater sense of spaciousness and well-being.

Boeing is delivering its Sky Interior design, complete with mood lighting and more comfortable coach seating, on new American and United 737-800s.

Southwest’s version is called the Evolve Interior. Delta’s 737-900ER aircraft, slated for delivery beginning next year, will feature the Sky Interior. The 100-jet order will replace older aircraft in the fleet.

Paying for personal space
Years ago, American Airlines tried to woo passengers with more legroom throughout the economy cabin on certain business routes. It didn’t work. Competitors lowered their fares, and that was that.

In a somewhat surprising twist, passengers are now showing their willingness to pay for extra legroom, particularly for transoceanic flights.

United was the first to offer Economy Plus back in 1999, using it both as an additional revenue stream and as a perk for its elite flyers.

Next up: Delta, AA
Delta launched Economy Comfort last year on transatlantic flights and threw in free alcoholic beverages as an additional enticement. The experiment worked well enough that Delta will expand the option to transcontinental flights (minus the beverages) beginning June 7.

American Airlines, no doubt watching this phenomenon with some bemusement, will begin installing a similar product called Main Cabin Extra this year.

Is the extra room worth the price? Participants in online forums for airline passengers agree, for the most part, that it provides just enough additional room to stave off claustrophobia and allow for some sleep on a long flight.

Staying connected aloft
For some travelers, nothing mitigates the agony of a middle seat better than a good dose of Facebook (or other online chatter and entertainment) – which is why airlines are investing in in-flight Wi-Fi.

Delta boasts the most aircraft, including regional jets, with Wi-Fi. American offers it on 767-200s and “select” MD-80 and 737 aircraft.

Southwest touts its egalitarian price of $5 for all flights, all devices. United is playing catchup – it will begin installing Wi-Fi midyear – and says it will be the first carrier to offer transatlantic Wi-Fi.

Helpful hint: Passengers who want a quick look at email or Facebook should check Gogo’s inflight offerings to see if a 15-minute Wi-Fi freebie is available for their handheld devices.

Let us entertain you
The airlines are working diligently to retire overhead monitors on transcontinental and transoceanic flights, but they are still to be found in many economy cabins.

Video on demand is becoming the new standard, however.

Delta’s expanded content includes 100 movies titles, 20 hours of HBO programming, eight hours of Showtime, 27 hours of TV choices, 4,200 audio selections, 16 radio channels and a new Sky Kids feature.

American is working with Gogo to install streaming video on its entire fleet by the end of the year. It allows passengers to stream movies and TV shows from an inflight library to laptops and some tablet devices. Didn’t have time to watch the whole movie? You can take it with you.

Feeding the masses
In the aftermath of 9/11, domestic coach meals were unceremoniously axed.

Airlines tried to fill the gap with food for sale, but they struggled with the concept. Without knowing how many passengers would buy food, or what sort of food they would buy, airlines never got the hang of how much of what to stock on an aircraft.

Some resorted to snack boxes whose high-calorie, high-fat foods had a long-enough shelf life to eliminate the need to toss unused portions when a flight was completed. But a meal in a cardboard box has limited appeal.

Food for thought
Inflight food for the masses may be the next frontier for innovation.

A look at KLM’s a la carte option provides a possible clue. On intercontinental flights from Amsterdam, coach passengers can pre-order three-course Dutch, Japanese, Indonesian, Italian or vegetarian meals for €12 to €15 (about $15.35 to $19.20).

The meals are currently only available on flights from KLM’s home base in order to ensure quality and freshness. It would be much more difficult to maintain standards in hundreds of destinations. But perhaps someday, somewhere, some enterprising company will figure it out.

Message for travelers
Both the network carriers that are investing billions in passenger amenities and the rock bottom low-cost carriers that take in billions for everything from printing a boarding pass to providing overhead bin space have a message for today’s travelers: You can have anything you like, but you’re going to pay for it – extra.

Technology Editor Michèle McDonald has reported on the airline industry for more than 25 years.