KATHMANDU: The crash-landing of a passenger plane at Kathmandu airport that killed 49 people has put Nepal's aviation safety record in the spotlight once again.

Air travel is popular in the poor Himalayan country, but its mountainous terrain, poor regulation and a lack of investment in planes and infrastructure have led to large numbers of accidents over the years.

Here are five things to know about flying in Nepal:

- Poor national record -

There have been 27 fatal plane crashes in Nepal in the past three decades -- an average of almost one per year, according to the independent Aviation Safety Network database.

More than 20 of these occurred in the last decade alone and seven killed 18 people or more.

All its airlines are barred from flying in European Union airspace.

The majority of crashes happened at small domestic airports where pilots negotiated small planes onto tiny strips of tarmac -- in some cases hewn into steep mountainsides.

But the worst have occurred at Kathmandu's single-runway airport, which services both international and domestic routes.

In 1992, two planes crashed near Kathmandu within just two months of one another, killing a total of 280 people.

- Capital overstretched -

Aviation experts say challenging terrain is a key reason for the high accident rate in Kathmandu.

Nepal's only international airport sits in a narrow valley 1,338 metres above sea level, meaning planes have a relatively tight space to turn in.

The airport also lacks the sophisticated radar technology found elsewhere in the world, forcing pilots to navigate by sight, known as a non-precision landing.

The airport has only one runway, so planes have to remain in long holding patterns in the air -- a problem that has only worsened in recent years.

Nepal Airlines pilot Vijay Lama said Kathmandu provided the "basic minimum" and congestion at the airport was "creating havoc".

- Lack of investment -

Investment in Nepal's aviation sector has failed to keep pace with the boom in commercial flights -- both international and domestic.

Tiny Nepal, with a population of just 30 million, has 11 domestic airlines, almost as many as giant neighbour India.

Plans to upgrade airports in major tourist destinations Pokhara and Lumbini for international flights have been in limbo for years.

- Mountain landings -

Although most of the biggest accidents have occurred in Kathmandu, most pilots acknowledge that the precipitous landing strips high in the Himalayas are the toughest to negotiate.

Lukla -- the gateway to Mount Everest -- has been dubbed the world's most dangerous airport, requiring the pilot to navigate through narrow mountain gorges before landing on a steep runway just 500 metres (yards) long with precipitous drops on either side.

In 2008 a passenger plane crashed on landing at Lukla, killing everyone on board except the pilot.

"The country's beautiful but rugged terrain makes the safety of air operations more challenging than in other areas of the world," says the International Civil Aviation Organization, a UN special agency.

- Lessons not learned -

Some experts accuse Nepal's civil aviation authority of failing to act on recommendations from past crash investigations.

Lama, the pilot, said investigations teams usually did not widely circulate their conclusions, making it difficult for the industry to learn lessons.

But UK-based aviation expert Andrew Blackie, part of a team that investigated a crash in Nepal's south in 2016, said resources were an issue.

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"In Nepal there are some very tough choices about where the government is spending its money... they are very aware that an expensive investigation means less money spent on other things," he said.