While the announced departure of Rossen Milanov at the end of the 2010-11 season no doubt is a disappointment to his admirers, it can't be a surprise: Inevitably, the longtime associate conductor of the Philadelphia Orchestra would at some point move on to a position of greater visibility.

In fact, he already has, with ongoing positions with Symphony in C in Camden, the New Symphony Orchestra in his native Bulgaria, and, most recently, the Princeton Symphony Orchestra, plus a budding opera career in Europe.

Of his departure, Milanov, 45, said this week: "I'd been thinking about it for some years. The question was when it's a good time to do it.

"I've always had four jobs at any given time, sometimes five, so it's not a question of me finally branching out to do something else. I felt that I had a responsibility to the orchestra to make sure that . . . I didn't create any additional burden."

He was alluding to the orchestra's leadership transition at the top levels: In recent years, the posts of top conductor, president and CEO, and board chairman all have changed hands. However, Milanov's departure after next season - his 11th - is by happenstance part of a separate, middle-level transition.

Assistant conductor Danail Rachev leaves at the end of the current season in August. And David Hayes, who has worked 15 or 16 weeks a year as a "cover conductor" as well as leading family concerts over the last decade, plans to decrease his commitment to the orchestra.

"There's always an ebb and flow to the staff conductors," Jeremy Rothman, vice president of artistic planning, said Wednesday. "Conductors have to find opportunities that continue to challenge them. . . . We encourage them to guest conduct [outside Philadelphia] . . . but the end result is that they make their own choices as to how to grow their careers."

In late 2008, at 38, Rachev was named music director of the Eugene (Ore.) Symphony Orchestra, which helped launch the career of Marin Alsop, now in Baltimore, among others.

Though Hayes has never had an official title with the orchestra, he is artistic director of the Philadelphia Singers - its resident chorus - and teaches at the Mannes School of Music in New York.

Milanov has been conducting opera in Berlin and Stockholm, recently toured with the NHK Symphony Orchestra in Japan, and took on the music directorship of the growing Princeton orchestra, encouraged by the community's taste for adventurous, contemporary programming. With that and his Camden commitment, he'll maintain his Center City residence, and is expected to make guest appearances with the Philadelphians.

Meanwhile, the orchestra is having extensive conductor auditions in June, which Rothman characterizes as "a chance for us to see new, great talents. A lot of conductors are eager to come to Philadelphia. We're very fortunate in that sense."

Staff conductors often do more observing than conducting - monitoring rehearsals and learning repertoire should they need to step in on short notice. Actual stage appearances are at less-visible concerts not part of the usual Kimmel Center subscription - an important learning experience, but not necessarily a satisfying position in the long term.

About 90 percent of Milanov's performances were based on a single rehearsal, often as part of the orchestra's neighborhood concerts or the summer seasons at the Mann Center and the Bravo! Vail Valley Music Festival. He did make important last-minute appearances at the end of Wolfgang Sawallisch's music directorship when the elderly maestro was too ill to perform.

Since Milanov often had the most populist concerts in his 18 weeks per season, he was the most consistent face of the Philadelphia Orchestra during a decade that included the tenures of Sawallisch (1993-2003), Christoph Eschenbach (2003-2008), and Charles Dutoit (2008 to the present).

He also was called upon to practice a skill not normally cultivated by young conductors, much less foreign-born ones like himself: talking to audiences. Even he was surprised at how naturally it came to him, particularly considering that he arrived in the United States as a rank-and-file oboist who morphed into a conductor during his years at the Curtis Institute and Juilliard School of Music.

The quality of his work has been remarkably consistent, even under rushed circumstances. "He's at least as good as 90 percent of the guest conductors we've had," said longtime violinist Davyd Booth. "He's incredibly efficient, he knows what he wants. And he always gave us complete run-throughs [of a program]. He's very good about budgeting his time."

"It's almost like working in an emergency room," Milanov said, adding that he was inspired by the orchestra's ability to play well in all settings.

Milanov had a subscription concert every year, and his taste in programming, it turns out, often reflected one of his reasons for leaving: He favors such less-than-popular works as Shostakovich's Symphony No. 15, as well as multidisciplinary pieces, which he wants to pursue in the future - not unlike the orchestra's collaboration with the Wilma Theater in the Tom Stoppard/Andre Previn theater piece, Every Good Boy Deserves Favor.

That 2002 project was also the scene of Milanov's closest brush with disaster, when during one Kimmel Center performance actor Richard Easton began forgetting his lines in what threatened to become a cast-wide epidemic.

Milanov had to feed the actors their lines one by one. "I tried to make it seem like it was part of the play," he recalls, "and some people thought that it was."

Contact music critic David Patrick Stearns at dstearns@phillynews.com.