# Aviation Accident Report: Pennsylvania Central Airlines Flight 19/Conclusion

IV.
CONCLUSION
The Board in this proceeding, in accordance with the statutory mandate, is reporting the probable, not the certain, cause of the accident. Undoubtedly the Board's statutory duty was thus defined in the Act in recognition of the fact, well known to the Congress, that due to the meager and inconclusive character of the evidence available, the circumstance surrounding air accidents have at times been enshrouded in obscurity. Probability flows from evidence which inclines the mind to a conclusion but leaves room for doubt. The Board in the present case is faced with just such evidence; evidence which suggests events but offers no basis for certainty with respect to them. Most of the subsidiary findings which follow, therefore, and certainly the conclusion as to the probable cause of the accident represent what appears to the Board to be the maximum of probability with respect to the several matters to which they relate. In some instances the conclusions lie in a twilight zone in which it has been extremely difficult to distinguish between probability and possibility.
Findings of Probable Fact
We find the probable facts to be as follows:
* The accident near Lovettsville, Virginia, on August 31, 1940, in which aircraft NC 21789 was destroyed and 25 lives lost, occurred at approximately 2:41 P.M. (EST).
* At the time of departure from Washington at 2:18 P.M., the aircraft, its equipment, and its personnel were in proper condition to undertake the flight, and all requirements of law, regulation, and company ​practice had been complied with.
* A mechanical adjustment made immediately prior to departure, to correct lowered oil pressure on one engine, was without significance with respect to the accident.
* The flight was dispatched in accordance with normal procedure, and all information in the possession of the Weather Bureau and the company meteorologists had been taken into account in connection with the dispatch.
* The weather forecast contained nothing which would normally have raised any question about the advisability of dispatching a regular airline operation.
* The weather forecast was made in as much detail as the present state of meteorological knowledge permits, and proved to have been substantially accurate except for the omission of any reference to extraordinarily heavy rainfall.
* Immediately prior to the accident the airplane was proceeding on its normal course, and at normal altitude, in accordance with the flight plan.
* The airplane began its descent in the immediate neighborhood of, and immediately after, an intense flash of lightning.
* There were several other lightning flashes within a distance of two or three miles, and within a short space of time before and after the accident.
* The thunderstorm was accompanied, or immediately followed, by rainfall of extraordinary intensity, the heaviest known in the neighborhood ​in several years.
* The descent of the aircraft began approximately at the time of its entering the area of intense rainfall.
* The air through which the airplane was flying at the time of the accident was turbulent.
* The airplane did not spin or undergo any substantial lateral deviation from its course during its descent.
* At some time during the descent the propellers were turning at substantially above their rated speed.
* The speed of the aircraft at the instant of striking the ground approached or exceeded 300 miles per hour.
* This speed could have been reached in a steady descent from cruising altitude along a path inclined 30 degrees to the horizontal.
* The aircraft at the time of take-off was loaded very nearly up to its maximum weight limit, but was clearly within that limit.
* The center of gravity of the airplane was in an intermediate position and the airplane would have had strongly positive longitudinal stability as a result.
* To maintain a path angle inclined as much as 30 degrees to the horizontal or any steeper angle, it would have been necessary to maintain a steady pressure of at least 40 pounds against the control columns, or to jam or block the controls in a fixed position.
* The altitude and position of the airplane at the time of the first indications of trouble was such that if the difficulty had been a power plant failure the pilot could have turned back completely out of ​the storm area and made a forced landing under reasonably good conditions.
* Power plant failure had no causative relation to the accident.
* There was no structural failure prior to striking the ground.
* Immediately upon impact the fuel carried in the aircraft (about 400 gallons at the time of striking the ground) caught fire and burned with great rapidity. This was accompanied by an immense burst of flame and the production of rapidly rising currents of air.
* Individual parts of the aircraft continued to burn for some minutes before being completely extinguished by the rain which had its full intensity at the point of impact.
* The papers and pieces of cardboard found at distances up to 1¼ miles back along the flight path from the point of impact were carried there immediately after the accident by violent rising currents of air caused, in large part, by fire following impact and a light westerly wind.
* There was no fire in the aircraft prior to impact.
* There was no sabotage.
* The airplane was not actually struck by lightning.
* The airplane was in some fashion affected, or the pilots disabled, by some effect incidental to a stroke of lightning, such as its mechanical effect on the airplane, or acoustical shock, concussion, or optical impairment of the pilots.
​In view of the absence of persuasive evidence that the accident was caused by structural failure of the airplane, mechanical failure of its motors, fire, heavy rainfall, or sabotage, we are left with turbulence and lightning as the two major possibilities on the present record. While it has been found that the airplane was flying through turbulent air at the time of the accident, it seems highly improbable that turbulence alone could account for the loss of 5000 feet before recovery of level flight. It is possible, of course, that involuntary interference by the jump seat occupant, who may have been thrown into the cockpit, could have accounted for the inability of the pilots to regain control once it had been lost. (The Board has under consideration a regulation prescribing the technical qualifications of any person who may be permitted to occupy the jump seat.)
Especially in view of the absence of persuasive evidence indicating a other probable cause of the accident, we are greatly impressed by the evidence of the coincidence of the lightning flash seen to be in close proximity to the airplane and the immediate descent of the airplane. Nor is this impression altered by the fact that all-metal aircraft are commonly struck by lightning with no injurious results and that the character of the lightning discharge, as well as its effect upon the airplane crew, in the present instance must be regarded as an extremely unusual occurrence.
Probable Cause
Upon the basis of the foregoing findings of probable fact and the entire record in this investigation, we find that the probable cause of the accident to aircraft NC 21789, which occurred at Lovettsville, Virginia, on August 31, 1940, was the disabling of the pilots by a severe lightning discharge in the immediate neighborhood of the airplane, with resulting loss of control.
Recommendations
* The possible effects of lightning upon aircraft should be the subject of continued research. The difficulties of direct research upon a natural phenomenon occurring unpredictably as to time and location, and only at considerable intervals in any particular area, are obvious. Nevertheless, much has been accomplished in the last twenty years in a number of laboratories. The National Advisory Committee for Aeronautics has for some time had a special subcommittee on Lightning Hazards to Aircraft, on which interested Government departments and airlines and industrial laboratories having given special attention to lightning are represented, and some specialized researches have already been conducted. We recommend that further attention be given to such research, and that there be included among other matters the study of the optical quality of lightning flashes, the nature of their blinding effect, and the extent to which protection can be given against blinding by the use of windshield materials of various light-transmission characteristics; the possibility of acoustic shock through radio earphones, and the extent to which the liability of such shock would vary with the particular characteristics of the earphones used and of other parts of the radio installation; and the possible mechanical effects of lightning on aircraft in flights.
* A continuation and accentuation of research on atmospheric turbulence. Here again, as in the case of lightning, direct research is difficult, but a substantial amount has already been accomplished. We believe it important that the existing store of knowledge of the nature of atmospheric turbulence, of its structure when its severity is at a maximum, and of its effect on the flight performance and the control of the aircraft should be extended as ​rapidly as possible. It is of particular importance that further knowledge should be sought on the probabilities of association of extreme turbulence with predictable meteorological conditions, so that the likelihood of such turbulence can itself be predicted. The development of a generally acceptable scale of measurement of the severity of turbulence, and of a standard nomenclature for the various types of turbulence, would also be valuable as a possible product of further study. It is hoped that research in this field may receive a high priority with the Weather Bureau, the National Advisory Committee for Aeronautics, and such other governmental and private agencies as may have special interest in the problem or special qualifications to deal with it.
* Methods should be developed for collecting and correlating the experiences that airplane pilots may have with exceptional turbulence, or other unusual atmospheric conditions. It happens from time to time that a pilot, military, airline, or private, observes some exceptional atmospheric condition, or some exceptional effect upon the flight of the aircraft. The ability of pilots to benefit from one another's experiences in such matters is quite largely limited to the results of informal report. It would be of great value in connection with continued research on atmospheric structure and its effect on aircraft if records of all such exceptional experiences could be brought into the hands of a single agency, for comparison and the development of such generalized conclusions as the collected mass of data may allow. The National Advisory Committee for Aeronautics has already made progress along this line, with specific reference to the magnitude of the accelerations of aircraft in turbulent air and to the conditions noted by pilots ​at the time of lightning strikes, but it would appear that a much more extended program of collection and correlation of data would be possible. It is recommended that this matter receive the consideration of all organizations operating substantial fleets of aircraft, and of the National Advisory Committee for Aeronautics, which already has in existence a sub-committee on meteorological problems on which the Administrator of Civil Aeronautics, the Weather Bureau, and the Army and Navy as the major aircraft-operating departments of the Government are all represented.
* Where pilots encounter conditions not shown in the latest weather report for the area or in any forecast, including those cases in which they encounter turbulence of what appears to them very exceptional intensity, their observations should be transmitted to the nearest available Weather Bureau office at the earliest possible moment. Not only would a better-established practice in this particular be of assistance in correcting and amplifying current reports for the benefit of other pilots and for the improvement of forecasts currently being made, but information received on current turbulence conditions should be of assistance in research on the forecasting of turbulence by making it possible to establish immediate correlations between turbulence data and other weather information which can be currently secured for the regions of reported exceptional turbulence.
THE CIVIL AERONAUTICS BOARD:
George P. Baker, Member