Indian authorities have been holding the six men since their arrest in December 1995 for dropping weapons and explosives in the West Bengal region reportedly to aid Hindu extremists.
Commander Alexander Klishin, pilots Igor Moskvitin and Oleg Gaidash, engineers Yevgeny Antimeko and Igor Timmerman and former British army officer Peter Bleach were convicted Jan. 31 on charges of illegal smuggling of arms and explosive substances, violation of aviation laws, false testimony and promotion of war.
Indian authorities made it clear since their arrest that if convicted the pilots could face a life sentence.
"We received the news with regret," said foreign ministry spokesman Janis Selis. "We believe the sentence is too harsh.
Of those convicted, only Moskvitin remains officially connected to Latvia. The other four from here applied for and received Russian citizenship last year.
Since being imprisoned Moskvitin has contracted tuberculosis, prompting the Latvian government to lobby for his release.
"We asked on humane grounds to liberate Moskvitin on grounds of illness," said Selis. "We have not yet received an answer."
All five plan to file appeals in the North Bengal Supreme Court this week.
While Latvian officials say they are powerless to change the sentences, they have pressed Indian authorities to sign an extradition treaty but the Indian ambassador to Latvia said in a meeting with foreign ministry officials in Stockholm last week that an agreement wasn't possible due to "legislative deficiencies."
"The problem is that the (Indian) foreign ministry cannot talk on this issue because the Indian parliament has not given them permission to talk with us," said Selis.
The crew members from Latvia are former employees of the defunct Latvian Airways and fell into trouble when they met Kim Davy, the man suspected of allegedly masterminding the drop who is now on the run from Indian authorities.
When Latvian Airways went out of business, Davy bought one of the planes and offered the former Latvian Airways crew members a contract to fly it to India.
After the drop in West Bengal, the plane landed in Bombay on Dec. 22, 1995 where the crew was arrested.