This article describes a voice-activated flight information and reservation system. The system is based on the speaker-dependent, real-time, large vocabulary speech recognizer developed at the IBM Rome Scientific Center. To remove the speaker-dependency constraint, a speaker-independent acoustic model was built. To achieve a high recognition rate, a technique was developed to introduce grammatical constraints into the probabilistic language model of the recognizer. In particular, instead of substituting the language model for a grammar, we trained it on the "complete" corpus of the language to recognize. The corpus was generated using a generative grammar. No grammar was used during the recognition process. The high recognition rate attests the validity of the approach.