Effective Date: 15 June 98

Takeoff Flight Path

This calculating problem seem to be only under the military operating rules. The FAA rules seem to be satisfactory. For this presentation, a four (4) engine aircraft was chosen.

Aircraft performance planning is normally based upon having an engine failure occur at some critical time during the ground roll and either aborting or continuing the takeoff. For the continued takeoff conditions, a takeoff flight path is provided base upon 3 engines for a four engine vehicle. Conventionally, the normal 4 engine takeoff flight path has been provided; at least in Military Performance manuals.

The capability of the 4 engine flight path may have lulled the performance analyst into a since of complacency by the use of vague words such as "the 4 engine flight path is always better than the 3 engine flight path." This statement is, of course, based on facts, such as, an additional engine increases the gradient by some percent. Then, the analyst proceeds to establish a 4 engine flight path where, by legislation, no engine failures are considered. This rule is carried into the initial enroute portion in the profile and will usually show good capability and very seldom will be limiting relative to the military Standard Instrument Departure (SID) procedures. The following paragraphs will present arguments for re-thinking of the 4 engine flight path ground rules and the use thereof with respect to the 3 engine criteria. Also discussed will be problems relating SID to operational performance and policies that can be derived.

Figure 1 shows the 3 engine, T. 0. Flaps takeoff flight path having an engine failure occur prior to lift-off. The requirement is to clear all obstacles within the 22.1 N. Mi. SID area.

Figure 2 shows the 4 engine takeoff flight path with gear retraction, flap retracting, acceleration to climb speed and enroute clime segments. This figure illustrates the SID clearance requirements are. Shown also is the 3 engine profile transferred from Figure 1.

Figure 1 and Figure 2 present the conditions normally specified. The obstacle at 22.1 N. Mi. is selected as the "controlling" obstacle in the area according to SID. This is suppose to be the most critical obstacle for a climbout measured from the end of the runway. It is, if the gradient is constant. The gradient for all aircraft is not constant, since configuration change, accelerations, etc. are considered in the profile.

To illustrate this, Figure 3 shows an extension of the 22.1 N. Mi. obstacle toward the end of the runway. This "smaller" obstacle, through critical to this particular 4 engine profile example, would not be shown on SID map. This is because SID lays a planar surface from the runway end to the obstacle which in turn determines the steepest gradient within the 22.1 N. Mi. area. This is the "controlling" obstacle. At anytime an aircraft flight path is below this gradient, it is unsafe.

Figure 4 shows this area of unsafeness in the profile presented thus far. The area (A) is unsafe in the 3 engine profile and the area (B) is unsafe in the 4 engine profile even though the aircraft clears the controlling obstacle by the SID criteria (100 feet per 1 N. Mi. up to 500 feet). The 3 engine unsafe area, (A) must be considered, however, in conjunction with the probability of combining an engine failure occurring exactly at Critical Engine Failure Speed on a takeoff having a down-range obstacle masking a critical close-in obstacle while operating at the maximum obstacle clearance gross weight. This does significantly reduce the possibility of the 3 engine case occurring. The 4 engine case does, however, remain unsafe anytime the close-in obstacle is above the flap retraction altitude. The method to make the 4 engine case safe at all times is to increase the flap retraction altitude such that the 4 engine case never falls below the gradient dictated by the SID controlling obstacle.

Before, however, this is done, we should consider the possibility of an engine failure at any other time in the flight path. No information of this type is normally provided making an evaluation of the seriousness of the problem when it does occur impossible. Figure 5 illustrates the flight path where an engine failure occurs at points other than before lift-off. In these cases failures at both Points (B) and (C) will place the aircraft below the down-range "controlling" obstacle. To eliminate this case which is more possible because increase exposure time, the flap retraction altitude is increased such that the 4 engine flight path where an engine failure occurs never falls below the 3 engine flight path.

To graphically illustrate this, the segment (A) and (D) in Figure 5 is translated such that the most critical point in the profile, in this case, Point (C), coincides with the 3 engine flight path, thereby showing the new flap retraction altitude. Though this illustration in Figure 6 shows that Point (C) is the most critical, it is altogether possible that, depending on thrust ratings, the acceleration segment length, windmilling drag, etc., that the down-range points may be more critical for the particular aircraft in question, such that all areas must be considered.

Much of the problem would be eliminated if the four segments of the flight path were supplied in the manual separately and the crew members allowed to compute the specific flight path. The flight path would, in all cases, have to be based on 3 engine operation. This does not eliminate the area of concern for the case where the 3 engine climb speed is lower that the 4 engine enroute climb speed such that the distance covered with 4 engines would be greater than on three.

The problem illustrated in the previous paragraphs show the need for a better definition of the flight path for both Military and Civil operation. Neither criteria require that an engine failure be considered at any point in the takeoff flight path that may be critical, though the Civil approach in using flight path segments does eliminate three quarters of the problem. In any case, an approach must be developed to protect the aircraft under normal 4 engine operation.