If this is May, it must be pumpkin-seed-encrusted chicken breast.

Or pesto grilled mahimahi. Or portobello mushroom stuffed with wild rice on a bed of sautéed spinach.

The advent of spring brings many seasonal imperatives. Birds fly north. Flowers bloom. Airline food changes — slightly.

Swap out the beef tenderloin with bernaise sauce, swap in the beef tenderloin with teriyaki jus. Orange chipotle chicken gives way to barbecue chicken. Out with the tortellini with vodka Alfredo sauce, in with the cavatelli with garlic olive oil.

Airlines are refreshing their menus for the busy summer flying season, wooing first-class and international travelers with fare as ambitious as cramped, low-humidity, low-pressure cabins will permit.

In coach class, passengers get a hot meal only if they cross an ocean, but even there, change is in order. Yesterday's danish on US Airways' trans-Atlantic coach service is gone, replaced by today's flattop muffin (blueberry for May, apple-cinnamon for June).

For all the airlines' efforts, high-altitude dining is not haute cuisine. It's burdened with limited expectations, for food and airplanes are not a natural pairing.

Almost all food is less flavorful at 35,000 feet, because a passenger's ability to taste suffers from the reduced cabin air pressure and, especially, the low humidity. It's like eating with a cold.

To make matters worse, airline food must be bland enough for a million palates, cheap enough for a miser's budget, and durable enough to survive precooking, chilling or freezing, and reheating.

It's also made in mass quantities and served in tight quarters.

"I don't view it as a meal, but as a miracle of biblical proportions," said Hector Adler, US Airways' vice president for in-flight services.

Unsurprisingly, the bar for success is not set too high.

"The key to being successful," Adler said, "is whether it's moist and has a discernible taste."

Last week, chefs for US Airways — Philadelphia International Airport's dominant carrier — gave flight attendants and airport employees here a taste of what's ahead, unveiling the season's new trans-Atlantic menus and dishing up samples of the first-class meals.

There was, as always, a steak.

The airline's signature meal is steak, appealing to American travelers' sense of familiarity and to foreign travelers' sense of real American food. All that ever changes is the sauce and accompanying vegetables.

"Passengers always tell us in surveys, 'We want healthy food,' but it seems they always go for steak and ice cream," said Timothy Donnally, the airline's manager of menu planning and galley design.

There was, as always, a chicken dish (pumpkin-seed-encrusted for May outbound passengers), a seafood dish (pesto grilled mahimahi with cherry tomato sauce), and a vegetarian dish (wild rice-stuffed portobello with roasted pepper sauce).

Like its competitors, US Airways lavishes its culinary attention on the front of the plane, where the most profitable seats are. For the 85 percent of the Europe-bound passengers flying coach, May's selections are orange chipotle chicken or penne with Sicilian tomato sauce. Because of price (the airline says each coach meal costs it about $6), beef is offered only in June (stroganoff, if you're planning ahead).

The guiding principles of coach food are: sauces to keep it moist, and salt to boost flavor. In first class, meat is often encrusted to keep it moist.

"We work a lot with herbs, and you have to have a little more salt upstairs than on the ground," said chef Jimmy Clack, the airline's senior specialist for menu design and development. "It needs to be full of flavor, but not overspiced."

Despite all the planning and testing and tasting, some meals are flops. Last year, a chicken entrée was too salty. And the penne arrabiata was too spicy, especially for young fliers and their parents.

"Don't worry, the passengers let us know," said Donnally.

Only one food, insisted Adler, is unchanged in taste or popularity by taking flight: "Ice cream. Ice cream is the only food that tastes the same at 35,000 feet. And everyone loves it."

Maria Sirabella, a Philadelphia-based flight attendant on trips to Italy, said coach passengers generally prefer meat to pasta and tend to be surprised that the food is palatable at all.

"They'll say, 'This was really good.' They seem to think it's going to be bad," she said.

No wonder. The science of eating at altitude does not suggest a happy outcome.

Paul Breslin, a professor in Rutgers University's department of nutritional sciences and a faculty member at Monell Chemical Senses Center in Philadelphia, said the low-humidity air in airliners affects passengers' olfaction, their sense of smell, which is crucial to perception of taste.

"When the air is very dry, the mucous membranes in your nose are dry, and olfaction doesn't work very well," Breslin said.

"When you react to food, you react to flavor, and that's all of the sensory systems coming together," he said, citing taste, smell, texture, temperature, and spiciness. "We're talking about how people relate to all of the sensory information from food."

Airline food often tastes like institutional fare because, well, it is.

The complex business of provisioning planes with beverages, snacks and meals involves massive catering kitchens, warehouses, and a parade of supply trucks, operating with militarylike precision and often with militarylike results.

LSG Sky Chefs, the catering conglomerate that handles food preparation for US Airways and other airlines at Philadelphia International, last year prepared nearly 500 million meals for 300 airlines in 52 countries.

In a commissary just off Island Avenue, 210 US Airways employees work two shifts a day, stocking carts that serve 30,000 passengers a day. They are required to put every Coke, every ginger ale, every Bud Light in exactly the same place in every compartment to make it easy for a flight attendant to locate in a dim cabin.

The cooked meals, for first-class domestic flights and all international flights, are brought from the neighboring LSG industrial kitchen and stored at the commissary or taken to waiting planes.

For the largest planes in US Airways' international fleet, that means 300 meals.

Guillaume de Syon, a history professor at Albright College who has studied aviation's long, checkered affair with food, said passengers want to eat on a plane, even if the food is not very good.

"We've wanted to fly for thousands of years, and now that we can, we get bored. The flight attendants bringing us our food and beverage is part of our entertainment," he said. "We rag on the meal, but we still want it."

Contact Paul Nussbaum at 215-854-4587 or pnussbaum@phillynews.com.