Violent eruptive processes on the Sun can lead to an acceleration of solar energetic particles, which accordingly result in notable space weather effects, specifically an increase of the complex radiation field at aviation altitudes. A specific class represents events observed by ground-based detectors such as neutron monitors (NMs), the ground-level enhancements (GLEs). Here, we considered for study a specific event, namely the strongest known, yet not directly observed, that is the extreme solar particle event of 774 AD discovered on the basis of cosmogenic-isotopes measurements. After a convenient scaling of a GLE $\#$ 5 and employing the corresponding radiation model we computed the ambient dose at aviation altitudes during the 774 AD event. Since the spectrum of solar protons during 774 AD can not be directly obtained, as a first step we derived the spectra of the solar protons during the GLE $\#$ 5, the strongest directly observed by NM measurements. The GLE $\#$ 5 is assumed as a conservative approach because of the hardest derived spectra. The global map of the ambient dose was computed under realistic reconstruction of the geomagnetic field during the 774 AD epoch, obtained on the basis of archeo-paleomagnetic measurements. We show that the 774 AD event represents a significant space weather threat and can be used as a reference for the worst-case scenario for radiation dose received during GLEs at aviation altitudes.