I fly both the MD-80 and MD-90 for SAS,and as Atlamt states,the typerating is the same.SAS policy is to check out pilots on the MD-90 after 1 year experience on the MD-80.Our cockpits are the "standard" MD-80 layout with partial glass cockpit,there is also a full glass cockpit option which is used on Saudia planes (called the "MD-11" cockpit BTW).The only problems SAS had when the plane was introduced in -96 was nothing but teething problems with the electronic caution systems.The V-2500 engines have proved to be the most reliable engines ever operated by SAS.Also,being FADEC controlled,they are a dream to operate.Hydraulic fluid seeping into the APU air intake is also unknown in SAS,there is a V-shaped device just ahead of the intake to deflect any leaking fluids and it works.
The MD-90 is a dream to fly,being just about everything the MD-80 should have been in the first place.There's a fully hydraulic elevator system iso. the MD-80's DC-9 type system,making for very smooth and precise control in the pitch plane.It is also VERY quiet,both in the cockpit,cabin and outside.The cabin interior in the SAS version is fairly similar to the MD-80,but there are several differences;the forward lavatory has 2 windows,MD-80's having been upgraded to the new SAS standard have only 1.In the MD-90,a seat has been removed adjacent to the forward overwing emergency exits.This means that if you are seated in seats 20A and 20F there is no seat in front of you before you reach row 18.In seat 19B you are all alone on the left side of the aisle.
One drawback with the -90 is that it utilizes the MD-80 wing which is a bit too small for the MD-90 weights and high altitude performance suffers a bit.
SAS has been looking for additional MD-90's for the past few years as the present fleet of 8 is too small for really economic operation.At least 15 aircraft in the fleet is desirable.Ex-Reno aircraft were not up to the technical standard desired,so they were dropped.Latest rumours are that MD-90's from one of the Chinese operators are being looked at.
The MD-90 entered SAS service by accident.When the downturn in the early -90's started,SAS had 18 MD-80's on order.Instead of paying the cancellation fee in much-needed cash,it was converted into an order for 6 MD-90's,the intention being that the planes would be leased to others when delivered.There were no takers,and when the planes were completed,SAS had a need for these planes themselves and ordered 2 additional planes,as well.
"Luck and superstition wins all the time"!