 The 747SP uses single-piece flaps on the trailing edges, rather than the smaller triple-slotted flaps of standard 747s.The SP could accommodate 230 passengers in a 3-class cabin or 331 in a (303 economy, 28 business) 2-class cabin, and a maximum of 400 passengers in one class.A special 747SP is the Stratospheric Observatory for Infrared Astronomy (SOFIA) astronomical observatory, which had its airframe modified to carry a 2.5-meter-diameter reflecting telescope to high altitude, above 99.9% of the light-absorbing water vapor in the atmosphere.