LRRP - Obviously a very alarming experience for you and your passenger. Only you can determine whether or not to file an AIRPROX by considering if in your opinion the incident fell within the definition.. .. .A couple of points to consider in addition to those mentioned by previous respondents.. .. .Except where aircraft are leaving controlled airspace by descent, ATC should not normally allocate a flight level or altitude to an aircraft which provides less than 500 feet vetical separation above the base of a control area or airway.. .. .This is specifically to provide some separation from aircraft operating beneath the base. Therefore in this case where the base of the London TMA is 2500 feet, the airliner should not have been below 3000 feet.. .. .Secondly, can you remember who`s QNH setting you were using. As I`m sure you know, the base of the London TMA is determined by the Heathrow QNH, although it is permissible to use the QNH of any aerodrome situated beneath the TMA as the differences are considered to be insignificant. Therefore if you were using the Biggin or Heathrow QNH then that is perfectly in order. . .. .There have, however, been incidents caused by aircraft flying close to the base of control areas using a Regional QNH even though the airspace beneath TMAs doesn`t form part of the Regional QNH system. The upshot of this has been that because a Regional QNH is a lowest forecast setting used for terrain clearance purposes aircraft have infringed the base and come close to traffic inside controlled airspace.. .. .I`m not suggesting for one moment that this is what happened but it`s just a thought.. .. .Happy flying from now on.. . . . <small>[ 27 March 2002, 17:41: Message edited by: Duke of Burgundy ]</small>