SB 4.9.56

prākārair gopurāgāraiḥ
 śātakumbha-paricchadaiḥ
sarvato ’laṅkṛtaṁ śrīmad-
 vimāna-śikhara-dyubhiḥ
Synonyms: 
prākāraiḥ — with surrounding walls; gopura — city gates; āgāraiḥ — with houses; śātakumbha — golden; paricchadaiḥ — with ornamental work; sarvataḥ — on all sides; alaṅkṛtam — decorated; śrīmat — valuable, beautiful; vimāna — airplanes; śikhara — domes; dyubhiḥ — glittering.
Translation: 
In the capital city there were many palaces, city gates and surrounding walls, which were already very, very beautiful, and on this occasion all of them were decorated with golden ornaments. The domes of the city palaces glittered, as did the domes of the beautiful airplanes which hovered over the city.
Purport: 

Regarding the mention of airplanes here, it is suggested by Śrīmad Vijayadhvaja Tīrtha that on this occasion the demigods from higher planetary systems also came in their airplanes to bestow their blessings on Dhruva Mahārāja on his arrival at the capital of his father. It also appears that all the domes of the city palaces as well as the pinnacles of the airplanes were decorated with ornamental work in gold, and, being reflected by the sunshine, they were all glittering. We can observe a specific distinction between Dhruva Mahārāja’s time and modern days, for the airplanes in those days were made of gold, whereas at the present moment airplanes are made of base aluminum. This just gives a hint of the opulence of Dhruva Mahārāja’s days and the poverty of modern times.