The first lighting used on an airport was during the latter part of the 1920s; in the 1930s approach lighting came into use. These indicated the proper direction and angle of descent. The colours and flash intervals of these lights became standardized under the International Civil Aviation Organization (ICAO). In the 1940s, the slope-line approach system was introduced. This consisted of two rows of lights that formed a funnel indicating an aircraft's position on the glideslope. Additional lights indicated incorrect altitude and direction.
When was the slope-line approach system introduced?
1940s

Some scholars note that Tibetan leaders during the Ming frequently engaged in civil war and conducted their own foreign diplomacy with neighboring states such as Nepal. Some scholars underscore the commercial aspect of the Ming-Tibetan relationship, noting the Ming dynasty's shortage of horses for warfare and thus the importance of the horse trade with Tibet. Others argue that the significant religious nature of the relationship of the Ming court with Tibetan lamas is underrepresented in modern scholarship. In hopes of reviving the unique relationship of the earlier Mongol leader Kublai Khan (r. 12601294) and his spiritual superior Drogon Chogyal Phagpa (12351280) of the Sakya school of Tibetan Buddhism, the Yongle Emperor (r. 14021424) made a concerted effort to build a secular and religious alliance with Deshin Shekpa (13841415), the Karmapa of the Karma Kagyu school. However, the Yongle Emperor's attempts were unsuccessful.
Who did the Yongle Emperor try to build a religious alliance with?
Deshin Shekpa

By summer 1918, a million American soldiers, or "doughboys" as they were often called, of the American Expeditionary Forces were in Europe under the command of John J. Pershing, with 25,000 more arriving every week. The failure of Germany's spring offensive exhausted its reserves and they were unable to launch new offensives. The German Navy and home front then revolted and a new German government signed a conditional surrender, the Armistice, ending the war against the western front on November 11, 1918.
What was the official name of the American army that went to fight in Europe in 1918?
American Expeditionary Forces