The captain of the FlyDubai jet which crashed in Russia last Saturday was due to leave the airline, citing fatigue, colleagues say.
The Boeing 737-800 crash in Rostov-on-Don killed all 62 people on board, including seven crew.
Pilots speaking anonymously to the BBC say fatigue was a contributory factor in the accident - claims FlyDubai says relate to "confidential information".
One pilot reported previously falling asleep at the controls from exhaustion.
"We are unable to disclose confidential information relating to our employees," a FlyDubai spokesperson said.
"It is important, not least out of respect for the families involved, that we do not speculate about the circumstances of this tragic accident, whilst the independent investigating authorities carry out their work."
The pilots who spoke to the BBC say their colleagues are at "significant and obvious risk" from fatigue. One of the sources has already resigned and another says he will quit.
Poor visibility and high winds are being considered as the cause of the accident, which occurred after the passenger jet missed the runway as it attempted to land.
Reports say the plane abandoned its initial attempt to land and circled for two hours before crashing at the second attempt.
The three FlyDubai staff members said that the captain, Cypriot Aristos Sokratous, had already resigned and was serving out his three-month notice, stating fatigue and lifestyle as his main reasons for leaving.
"This crash was very close to home," a FlyDubai pilot told the BBC. "I don't want to speculate on what caused the crash, but I think that fatigue must have been a contributory factor. I'm also not surprised it happened.
"Crew are overworked and suffering from fatigue. It is a significant risk.
"Staff are going from night to day shifts without enough rest in between. I would say 50% of the airline's workforce are suffering from acute fatigue.
"I raised it with a senior member of staff at the airline who said 'we don't have a fatigue issue at FlyDubai'."
Pilot fatigue has long been stated as a concern in the airline industry. The European Aviation Safety Agency (EASA) has previously proposed setting limits on the duration that pilots can fly.
"The attitude among crew before this crash was that it's a case of when, not if, there would be an accident," one of the pilots said.
"We were asking 'do we have to lose an aircraft' for things to change? So staff, while shocked, are not surprised.
"The writing was on the wall. Most crews are running on empty anyway. Everyone is tired.
"I am worried, but I wanted to speak out. Why should people have to lose their lives?
"Approximately 25 pilots out of 600 have resigned since the beginning of the year. From my understanding, most have cited fatigue, rosters and quality of life."
Another pilot who spoke to the BBC said he and colleagues had also raised the issue of fatigue with the airline's management and he had admitted falling asleep on the flight deck on one occasion due to exhaustion. He said he is going to resign from the airline.
"The degradation in performance is noticeable," the second pilot said.
"I have fallen asleep at the controls due to fatigue. I also didn't have full mental capabilities on approach, which is incredibly serious.
"I admitted it and raised it with senior staff but nothing was done about it.
"More of the travelling public should be questioning what's going on at the front of the airplane and should know more about the welfare of the people flying the plane.
"I am resigning because of constant fatigue. I am going to give my notice in the next couple of weeks."
In a further statement the airline said: "For FlyDubai the safety and welfare of our flight crew and cabin crew is of primary importance. The whole aviation industry is heavily regulated.
"We strictly follow authorised flying duty time regulations in compiling duty rosters, with special attention paid to the variables which affect our crews including report times, previous duty and the number of days off.
"If a member of flight crew feels that, for whatever reason, they have not been able to get enough rest before starting a shift, our Safety Management Systems (SMS), encourages pilots to declare themselves unfit to fly."
Fatigue has been linked to a number of previous high profile aviation incidents. It was listed among the causes of a TransAsia flight which crashed in a heavy storm on Taiwan's Penghu island July 2014, leaving 48 dead.
It was also cited as causing a pilot to send an Air Canada passenger plane into a dive over the North Atlantic in 2011, injuring 16 people.
Officials say the cockpit voice and data recorders recovered from the crash scene have been badly damaged and are unlikely to reveal much data.
FlyDubai is a low cost carrier airline, which launched in 2009. It has a hub in Dubai and operates flights to some 90 destinations.
- 19 March 2016