# Starting CDM & handing over master

<p id="bkmrk-before-using-cdm">Before CDM can calculate any TSATs, one controller must be <strong>master</strong> for each airport. This page explains who that is, how to start CDM and how to hand over.</p>

<h2 id="bkmrk-who-is-master">Who is master?</h2>
<p id="bkmrk-only-one-controller-">Only <strong>one</strong> controller per airport can be CDM master, and only the master calculates TSATs. All other controllers are slaves by default and see the master's times.</p>
<p id="bkmrk-the-master-shall-be" class="callout info">The master shall be the <strong>lowest online position</strong> at that airport: DEL &gt; GND &gt; TWR &gt; APP/CTR (top-down).</p>
<table id="bkmrk-master-examples" style="border-collapse: collapse; width: 100%;" border="1">
<thead>
<tr><th>Online at BIKF</th><th>Master for BIKF</th></tr>
</thead>
<tbody>
<tr><td>BIKF_DEL, BIKF_TWR, BIKF_APP</td><td>BIKF_DEL</td></tr>
<tr><td>BIKF_TWR, BIKF_APP</td><td>BIKF_TWR</td></tr>
<tr><td>BIKF_APP only (top-down)</td><td>BIKF_APP</td></tr>
<tr><td>BICC_FSS only (top-down)</td><td>BICC_FSS</td></tr>
</tbody>
</table>

<h2 id="bkmrk-starting-cdm">Starting CDM</h2>
<p id="bkmrk-if-you-are-the-lowes" class="callout success">If you are the lowest position at an airport, write <strong>.cdm master ICAO</strong> (e.g. <strong>.cdm master BIKF</strong>) in the EuroScope command line.</p>
<p id="bkmrk-covering-several-air">If you cover several CDM airports top-down, write the command once for each airport, e.g. <strong>.cdm master BIKF</strong> and <strong>.cdm master BIRK</strong>.</p>
<p id="bkmrk-cdm-airports">CDM airports in BIRD: <strong>BIKF, BIRK, BIAR, BGGH, BGSF, EKVG</strong>.</p>
<p id="bkmrk-"><a href="https://wiki.vatsim-scandinavia.org/uploads/images/gallery/2026-10/6Ucimage.png" target="_blank" rel="noopener"><img src="https://wiki.vatsim-scandinavia.org/uploads/images/gallery/2026-10/scaled-1680-/6Ucimage.png" alt="Entering .cdm master BIKF in EuroScope"></a></p>

<h2 id="bkmrk-handing-over-master">Handing over master</h2>
<p id="bkmrk-when-a-lower-positio">When a lower position logs on, or the current master logs off, master must be handed over:</p>
<ol id="bkmrk-handover-steps">
<li>Agree the handover with the other controller.</li>
<li>The old master switches to slave by writing <strong>.cdm slave</strong>.</li>
<li>Once there is no master, the new controller writes <strong>.cdm master ICAO</strong>.</li>
</ol>
<p id="bkmrk-never-have-two-maste" class="callout warning">Never have two masters for the same airport. Coordinate the handover before typing the command.</p>
<p id="bkmrk-logging-off" class="callout info">If you are master and log off with no one taking over, write <strong>.cdm slave</strong> before disconnecting.</p>