Event and Flight Sequence
On 14 September 2005, at 21:15 local time, a Gulfstream G-V and a Boeing 737-300 experienced a loss of separation near Frankfurt/Main. The Gulfstream was on an IFR flight from Rotterdam to Stuttgart, while the Boeing 737 was operating a scheduled flight from Munich to Cologne.
The Gulfstream had been cleared by sector FFML to descend to flight level (FL) 250 at a rate of 1500 ft/min or more. The pilot subsequently contacted sector MAN, reporting a descent to FL150. The sector MAN controller, expecting the aircraft to level at FL250 based on coordination agreements, did not detect the discrepancy. The Gulfstream continued descending through FL240, where the Boeing was cruising at FL240 on a nearly opposite course.
Communication and Clearance Misunderstanding
According to the report, the Gulfstream pilot likely confused the descent clearance (FL250) with the descent rate requirement ("fifteen hundred or more"), leading him to believe he was cleared to FL150. The initial radio call to sector MAN used incorrect phraseology ("Rhine Control" instead of "Langen Radar") and ambiguous numbers, making it difficult for the controller to understand. The controller did not verify the cleared altitude.
Separation and TCAS Alerts
At 21:15, the controller noticed the Gulfstream deviating from the clearance and instructed it to climb back to FL250. The Gulfstream crew complied immediately. Meanwhile, the Boeing 737 crew received a TCAS resolution advisory to climb, which they executed. Short-term conflict alert (STCA) was active from 21:15 to 21:16. The closest point of approach was 0.7 NM laterally and 300 ft vertically. Both aircraft continued to their destinations without damage.
Findings and Cause
The investigation concluded that the prescribed separation minima of 5 NM laterally or 1000 ft vertically were significantly undercut. The cause was determined to be error-inducing communication. The Gulfstream's TCAS apparently did not exchange data properly with the Boeing's TCAS, as no resolution advisory was generated for the Gulfstream, and the Boeing's TCAS treated the Gulfstream as a Mode C intruder.