The general objective of most T-28 projects was to obtain data within and in the immediate vicinity of thunderstorms and hailstorms. The main emphasis of the research has been on the study of mature hailstorms. A typical scientific objective of the mature storm studies was to locate and characterize the precipitation growth regions for different types of storms. This involved determining the hydrometeor types and size distributions in various parts of the storm at various stages of storm development. A combination of aircraft, radar, and other data was also used to estimate precipitation growth trajectories.
For these investigations, a variety of flight patterns for penetrating mature storms evolved over the years. The basic procedure was that a qualified meteorologist with access to a quantitative weather radar system and real-time aircraft flight track data was in charge of vectoring the aircraft into desired areas of a storm. The vector selected normally permits penetration of a high radar reflectivity zone as well as an updraft region at aircraft altitude. Penetration of adjacent feeder clouds or other regions of the storm was important in some cases. A penetration was normally made at a constant heading until clear air was encountered or the T-28 was well clear of any radar echoes. The aircraft then reversed course in preparation for another penetration.
A limit was normally imposed on the maximum radar reflectivity factor permitted for penetration. This reflectivity limit was based on coordinate analysis of the experience from past penetrations along with radar reflectivity data and hailstones collected at the ground. The normal limit was 55 dBz along the penetration path at or above the altitude of penetration. This criterion was selected to permit the aircraft to encounter hail, but to avoid hail of destructively large sizes which would tend to damage the instruments and thus prevent the collection of the desired data. The maximum hailstone size encountered using this limit was normally about 2.5 cm, but larger particles were sometimes found.
A normal flight consisted of from three to six storm penetrations, although more than 10 have been made on some flights. This was limited mainly by the fuel supply (about 2 hrs with reserves). In the early years, the normal operational procedure in hail research programs was to begin penetration at 7.3 or 6.7 km MSL (24,000 or 22,000 ft MSL) and proceed downward at 0.6 km (2,000 ft) intervals on successive penetrations until 4.9 km (16,000 ft) was reached. This routine was interrupted on occasions when airframe ice built up to a point where the pilot considered another penetration unwise. Further penetrations were then delayed until a descent was made to melt the accumulated ice.
The pattern involving successive penetrations at progressively lower altitudes used in the early studies (e.g., Sand and Schleusener, 1974) was abandoned because of the difficulty of sorting out temporal from spatial (vertical) differences found in the data. More recent projects generally involved making repeated penetrations at a single level for each storm, usually in the temperature range from -5.0 to -15.0C where ice processes are believed to become operative in the storms. This indicates the evolution of each storm at the chosen level, and by studying other similar storms at different levels inferences can be drawn about the vertical structures. When additional observations are available from other aircraft penetrating the clouds at other altitudes, or operating below cloud base, information about the vertical structure is more readily obtained. Scientific questions about coalescence and recirculation processes, and unique radar signatures associated with melting precipitation, have suggested greater emphasis on storm penetrations near the 0.0C level, and some recent projects have included penetrations near that level.
The T-28 typically operated in projects involving other aircraft and often became involved in simultaneous penetrations of the same storm by two or more aircraft. This type of study has proven highly successful. Due to the workload in the T-28 cockpit, overall coordination of multiple aircraft missions had to be carried out from the ground or from another aircraft with sufficient crew.
The problems of greatest concern during penetrations were the intense turbulence encountered in some storms and the occasional occurrence of extremely rapid ice accumulation on the airframe (sometimes at an observed rate of 2.5 cm/min). The latter had the effect of rapidly increasing the weight and changing the aerodynamics of the T-28. When the air intakes to the carburetor and oil cooler were constricted by ice accumulation in such icing situations, a loss of power and high oil temperatures also resulted. The carburetor intake is protected from large hail ingestion by the grate described earlier, and this grate provides a surface area for heavy ice accumulation in regions of the cloud with high supercooled liquid water concentrations. Limited airflow to the carburetor was partially restored with the addition of carburetor heat; this procedure used air from inside the cowling, but could not restore full engine power. Even so, the engine occasionally stopped running due to excessive ingestion of ice and water. Fortunately, engine restarts were accomplished and there was no loss of data continuity in such cases.
It took about 20 min to climb to, or descend from, the normal T-28 operating altitudes (~18,000 ft MSL), leaving typically 80 min on-station time. This is less than the lifetimes of many thunderstorms, so judicious timing of the initiation of T-28 flights was important. Data tapes couldn't be changed in flight, but total data storage capacity was ~40 MB on the most recent data system. Typically, only ~20 MB was used even on the longest flights, so this was not a limiting factor.
Because of the adverse conditions in which the aircraft operated and the comparatively abusive treatment that resulted, high quality maintenance was imperative. This was provided by the employment of a mechanic whose primary responsibility was the meticulous care of the T-28. Normal maintenance included a very detailed inspection before and after each research flight by both the pilot and the mechanic. The engine was checked at regular intervals to detect any possible damage from rapid heating and cooling during penetrations. An oil sample was subjected to spectrometric analysis for metals at regular intervals to check for any abnormal internal wear. Only knowledgeable and qualified personnel were allowed to work around the aircraft and all work was checked and double checked. All modifications on this restricted-category aircraft were approved by the Federal Aviation Administration. The mechanic also conducted a regular program of progressive inspections to comply with FAA mandates concerning airworthiness.
For field operations, the T-28 facility required hangar space with sufficient power (about 20 A at 110 V) to run the aircraft systems (using our own 28 V power supply). The height clearance and floor space required was approximately 14 ft x 40 ft x 40 ft. Space required for tools and materials for in-field maintenance was an additional 20 ft x 20 ft. Separate space was generally required for the quick-look data reduction activities.
Minimum runway length was 4,000 ft. Other requirements were one hundred octane aviation gasoline, deicing alcohol, engine oil, and breathing oxygen.
The aircraft's VHR radios covered the band from 118.0 to 135.975 MHz. A dedicated project frequency for use between a meteorologist on the ground and the aircraft in flight, in addition to the frequencies normally used for conversations with FAA Air Traffic Controllers, was also required for T-28 flights.