Executive director is a term sometimes applied to the chief executive officer (CEO) or managing director of an organization, company, or corporation. It is widely used in North American non-profit organizations, though in recent decades many U.S. nonprofits have adopted the title president or CEO.

See e.g.:

http://en.wikipedia.org/wiki/Acting_(law)

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14. Op 10 maart 1997 zond de Commissie het ministerie een interne nota van het hoofd van de bevoegde dienst van het directoraat-generaal Financiële Controle aan de dienstdoende directeur van de bevoegde directie van het directoraat-generaal Landbouw.
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14. On 10 March 1997, the Commission forwarded to the Ministry an internal note from the head of the relevant unit in the Directorate-General for Financial Control to the acting director of the relevant directorate at the Directorate-General for Agriculture. (CELEX Documents of the European Court of Justice pt2.tmx)

Het agentschap dat bevoegd is voor de tenuitvoerlegging ervan, werd opgericht en een dienstdoende directeur is benoemd.
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The agency responsible for monitoring its implementation has been established and an acting director has been appointed. (CELEX External relations pt3.tmx)

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acting:
doing a job for a short time while the person who usually does that job is not there:

acting chairman/director/president:
Fisher will serve as acting director while a search is under way to name a permanent director

(http://dictionary.cambridge.org/dictionary/business-english/... )

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http://forum.wordreference.com/showthread.php?t=414917

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Note added at 11 mins (2014-09-26 10:53:49 GMT)
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‘In law, when someone is said to be acting in a position it can mean one of three things.

• The position has not yet been formally created.
• The person is only occupying the position temporarily, to ensure continuity.
• The person does not have a mandate.

The term "acting" is often used in one of these senses to refer to a temporary occupant of an office in government. An "acting" official holds office to ensure both the stability and continuity of his department will continue despite the absence of a formal leader.

For example, if the U.S. Secretary of Defense died suddenly in office, the U.S. Deputy Secretary of Defense would take over. However, he would only be "acting" in the position, as he/she will not formally hold the office unless he/she is nominated by the President and confirmed by the United States Senate, as required by the Constitution.

Acting officials typically play a caretaker role while acting, as it is usually considered questionable for someone to exercise full authority in a very activist way without having been specifically hired or elected to the office.

Acting for has the same basic meaning as "acting", except it indicates that the original occupant of the position still formally holds power.

For example, in 2006 when Israeli Prime Minister Ariel Sharon had a severe stroke, Deputy Prime Minister Ehud Olmert assumed power on the basis that he was "acting for" the incapacitated Sharon. Sharon was still formally the nation's leader, as he had not died or resigned, but Olmert was executing the powers of the office.’

(http://en.wikipedia.org/wiki/Acting_(law) )

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Note added at 7 hrs (2014-09-26 18:03:49 GMT)
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PLEASE NOTE:

In Rik's context (‘the person who is available at the time of a crisis’), Willemina's answer is obviously better. In other contexts, mine would be more appropriate.

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