1338. Operational Responses to Aft Empty C. G.

Paper

Abstract

When the 727-200 series, herein referred to as the Stretched airplane, came on the United Airlines scene in 1968, it became apparent that a ground handling problem of a magnitude not previously encountered was with us.
The sequence of events appear to be as follows:
A. An existing tail mounted engine design was stretched.
B. Since the centroid of the cabin and pits in this design is forward, the Stretched airplane is more tail heavy when empty and tends to be more nose heavy when loaded for flight, compared to the standard or -100 airplane.
C. Geometry, or location, of the main landing gear was unchanged.
There was no particular problem with the loaded airplane, even in ferry flight operation, since compliance with the aft CG limits and the normal landing fuel load produced acceptable, or at least unreported difficulties with nose wheel steering for taxi-in.
Off loading/loading of the airplane at the gate was protected by the requirements that the aft airstair must be down and locked before the airplane could be worked. The same rule applies to the case where maintenance is being accomplished.
The problems lay in the case where the empty or maintenance airplane was being towed or taxied and in the overnight airplane where it was necessary to park the airplane, aft airstair up, for security purposes. This was addressed by imposing a minimum normally distributed fuel load or fuel distribution requirements that would load the nose wheel.
The so called Advanced 727-200 airplane, which United Airlines began to receive in late 1977, worsened the situation. Further development of the design had produced the following changes.
A. -15 engines with soundproofing and quiet nacelles added approximately 1000 pounds at the back end.
B. Redesign in order to minimize weight growth results in a more tail heavy airplane since the weight removed tends to be at cabin centroid, forward of the main landing gear.
The net effect was that the empty CG of the Advanced airplane is some 3 to 4 inches further aft than the Stretched airplane. The ballast required to get inside the aft CG limit in ferry flight operation and the minimum fuel required for ground handling is discouraging. Where we had apparently learned to live with the Stretched airplane, the dam burst when the requirements of the Advanced airplane became known in the field.
A new problem arose in that the Flight Department now began to complain about the taxi-in characteristics of the airplane, particularly under slippery runway conditions. Ramp personnel also stated that towing-in was difficult at some tow-in gate positions due to nontracking of the nose wheel.
Out of all of this grew procedures to routinely utilize ‘payload fuel’ and/or what was then called ‘landing fuel credit’ in ferry flight operation. Additionally, a ground CG limit was introduced, more restrictive than the aft flight CG limit, to force load on the nose wheel and further increase the ballast requirements. Evolution of terminology, coordination between departments, and training were initial problems, that seem to have been essentially resolved with time.