Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/16

- 16 - Board does not believe that the 80-foot error resulting from the setting of the captain's instrument is chargeable to misreading of the instrument.

One other peculiarity of this instrument, however, has raised some question as to susceptibility of misinterpretation. On the right side of the instrument face a cutout is provided through which may be seen the drum which indicates 1,000-foot levels. On both sides of this cutout there has been printed a luminous triangular shaped index against which the 1,000-foot calibrations are read. In a darkened cockpit, the index which appears on the left side of the drum assembly cutout has at times been mistaken for the small 1000-foot pointer which is installed on the older altimeters. Such an error would leave the impression that a small hand was indicating an altitude of 2,500 feet. This fact had boon brought to the attention of the company prior to the accident and it had been agreed that the left index should be removed in order to prevent such a confusion; one of the company's fleet of six Electras had had this index removed at the time of the accident. The altimeters installed in the aircraft all contained indices on both sides of the cutout.

The Board is of the opinion that confusion is also possible in mistaking the right index for the 100-foot pointer. Although this may appear remote because of the distinctiveness of shape of the 1,000-foot altitude index as compared with the 100-foot pointer, these distinctions lose most of their significance at night, especially when hurried references to slight instruments are required in critical flight situations. If, in a hurried glance, the right index were to be mistaken for the large 100-foot pointer, the pilot would have the impression of being at 250 feet when, in fact, the aircraft might be considerably below this altitude.

As a part of the Electra training program, the company gave special attention to the need for training in the Bendix Flight Director System which was installed in the Electra and was otherwise new to the line pilots. The ground trainer in which the captain received approximately five hours of initial training on the Bendix Flight Director System had installed the conventional three-pointer altimeter and not the drum-type altimeter which was actually installed in the Electra.

The instantaneous vertical speed indicator installed in the Electra does not possess the lag typical of older instruments which rely solely upon a calibrated flow from the diaphragm for the initial indication of climb or descent. In this regard it can be stated that this type vertical speed indicator possesses characteristics which are definitely superior to those of older types. In at least one respect, however, the difference in presentation met be regarded as significant so far as this accident is concerned. This instrument installed in the Electra is calibrated in such a manner that a given displacement of the needle represents a rate of climb or descent almost three times as great as that shown on former designs. For instance, were the needle of the older instrument displaced 90 degrees downward from its normally horizontal position, it world signify a rate of descent of approximately 7§0 feet per minute. This same relative position in the case of the Electra instrument would signify a rate of descent of approximately 2,300 feet per minute.

In this connection, it should be understood that a proficient instrument pilot typically accomplishes more than 100 visual fixations per minute while flying solely by instruments during a maneuver such as an instrument approach. Any one fixation upon a flight instrument for the purpose of establishing a particular condition of flight such as airspeed, altitude, or rate of descent is generally a small fraction of one second. For the experienced pilot, therefore, it is common to rely