Those of you who have actually been paying attention for the duration of P-C will remember that Scootaloo and Rumble both have a condition called Aerodynamic Suspension, or AS.
 
Aerodynamic Suspension is an incurable magical disease that defects and heavily deteriorates the flight capabilities of a pegasus. There is virtually no treatment, except for an emergency wing reconstruction that attempts to clear and repair the severed magi-nerve lines. Only facilities in Canterlot and Manehatten have the equipment unnecessary to preform this surgery. The treatment is highly experimental and can only be preformed in emergencies, or if enrolled in a medical trial for magical diseases. Flight time may be increased up to 400% of the original time.