An airline pilot was refused permission to go part-time to care for her child because of a lack of resources, her employers at British Airways claim.
Ms Starmer wants more time to look after her one-year-old daughter
Jessica Starmer, 26, from Wareham in Dorset, is claiming indirect sex discrimination at a tribunal.
Senior BA manager David Warner said the Airbus fleet Ms Starmer flew with had a shortage of captains, especially co-pilots, when she made her request.
She was one of three women who asked for 50% working, but were offered 75%.
BA also said there were concerns about a relatively inexperienced pilot cutting their hours in half so early in their career.
Ms Starmer earlier said she would have to leave the job she loved if BA did not grant her request. She is also claiming loss of earnings.
She told the tribunal the company's "family-unfriendly working practices" reinforced male-dominated traditions.
When the first officer asked for part-time working Mr Warner, BA's senior manager short-haul at the time, said the resources "made it absolutely clear that (working a) 50% (contract) was not possible".
He told the hearing on Tuesday that during 2004 Airbus pilots had consistently raised concerns about their workload.
The cost of training a new pilot would be £45,000, plus ongoing costs of £8,000 to employ two pilots instead of one, he added.
And he said more pilots would be needed when 10 new aircraft came into service in the coming months.
BA's general manager of flight operations Paul Douglas, a former Concorde captain, told the tribunal Ms Starmer's application had triggered alarm bells.
He said: "I was very concerned that we were considering the possibility of allowing a pilot who was at an exceptionally early point in her experience to reduce to a 50% contract."
"My immediate response was that this was not something that the management team could be confident would be safe, and that approving it would put the integrity of our operation at risk."
Ms Starmer joined BA in May 2001.
He said he drafted a policy document in 2004 on part-time working setting out minimum hours that entry pilots, like Ms Starmer, must complete before they could reduce their flying times to less than 75% of full-time hours.
He denied the suggestion, made by Ms Starmer's counsel Michael Ford, that the policy was only drawn up to stop her from working at 50% full-time.
Earlier retired BA pilot John Rhodes, and representative of the British Air Line Pilots Association (Balpa), said he had helped draw up a previous part-time working agreement with BA.
A working group on it was set up after two women pilots put in a request for 50% contracts and threatened to take the airline to an industrial tribunal if it was refused, he claimed.
But Mr Douglas denied this, saying Balpa's involvement was limited to scheduling issues.
Ms Starmer told the tribunal earlier that she and her husband worked "extremely irregular" shift patterns, which made it difficult for them to care for their daughter. Her husband Simon is also a BA pilot.
The hearing was adjourned until Wednesday.