Savoia-Marchetti S.73

Picture - Czech Airlines S.73

Role: Airliner
Manufacturer: Savoia-Marchetti
First flight: 1934
Primary users: Ala Littoria Regia Aeronautica
Variants: Savoia-Marchetti SM.81

The Savoia-Marchetti S.73 was an Italian three-engine airliner that flew in the 1930s and early 1940s. The aircraft entered service in March 1935 with a production run of 48 aircraft. Four were exported to Belgium for SABENA, while seven others were produced by SABCA. The main customer was the Italian airline Ala Littoria.

Design and development

The aircraft was developed in only four months, thanks to the use of the S.55 wing, combined with a much more conventional fuselage. Developed in parallel with a bomber version (the SM.81 Pipistrello) the prototype S.73 first flew on 4 July 1934 from Cameri, with Adriano Bacula as test pilot.

The prototype had a four-blade wooden propeller on the central engine, and two-blade wooden propellers on each wing engine. Later all aircraft were fitted with three-blade metal propellers.

Picture - Savoia-Marchetti S.73

The S.73 was a mixed-construction (a skeleton of steel covered by wood and fabric for the fuselage, wood for the three-spar wing) monoplane with a braced tailplane and fixed undercarriage. There were two generators, one in each side of the fuselage; the batteries were 24 V and were rated at 90 A.

The pilot and co-pilot were seated side-by-side in an enclosed cockpit, with a compartment for a radio operator and a mechanic. A passenger compartment could house 18 passengers in two rows.

It had eight metallic fuel tanks, all in the wings, with a total capacity of 3,950 L (1,044 US gal). The prototype had French Gnome et Rhx´ne Mistral Kfr engines, but further aircraft had 522 kW (700 hp) Piaggio P.X Stella, 574 kW (770 hp) Wright R-1820, 544 kW (730 hp) Walter Pegasus III MR2V and Alfa Romeo AR 125 and 126. Propellers were three-blade, aluminium-steel variable pitch (only adjustable on the ground).

It could be used from small airports and had reliable handling controls, and was not too costly. The power on board was incremented with the latest types of engines, including Piaggio P.X, 522 kW (700 hp), Wright R-1820, 574 kW (770 hp), Walter Pegasus, 544 kW (730 hp), and Alfa AR.125/126. With the R-1820 engine it had 1,723 kW (2,310 hp) and a speed cruise/max of 270/340 km/h (170/210 mph), 1,000 km (620 mi) range, and 6,300 m (20,670 ft) ceiling. Interestingly, with the less powerful AR.126 it had 1,678 kW (2,250 hp), 345 km/h (214 mph), 1,000 km (621 mi) range, and 7,000 m (22,970 ft) ceiling. The SABCA license-produced aircraft had 671 kW (900 hp) Gnome-Rhx´ne engines for a total of 2,013 kW (2,700 hp), comparable to the last models of S.79s or the CANT Z.1018.

This aircraft was from the start a valid project, and few modifications were recommended by Regia Aeronautica. It was easy to fly, rugged, and easy to operate on the ground, including the ability to fly from short airfields and with bad terrain conditions, in spite of its relatively underpowered propulsion system and the lack of leading edge slats. Its mixed construction and fixed landing gear were its main shortcomings, when in the USA and Germany there were already full metallic aircraft. Some of these were faster or had better performance, but the S.73 was competitive with them for some years.

Operational history

In December 1935, an S.73 was used for a journey from Italy to Asmara, delivering over 200,000 letters, with 6,600 km (4,100 mi) traveled in four days, followed by the return trip to Rome on 6 January 1936. A commercial line was established covering a 6,100 km (3,790 mi) journey. SABENA had a similar route to Congo, with four days and 44 flying hours. The S.73 flew in passenger service with airlines such as SABENA, Ala Littoria, Czech Airlines and Avio Linee Italiane.

At the outbreak of World War II the aircraft was already obsolete, but some of the planes were pressed into service with Italy and Belgium as military transport aircraft. The military SM.81 variant served as a bomber, transport, and reconnaissance plane. It was operational in Abyssinia and Spain.

Five S.73s were present in Eastern Africa and used as military transports. The five S.73s were I-GELA, I-NOLA, I-NOVI, I-ARCO and I-VADO. I-GELA was destroyed by bombers, and I-NOLA was destroyed by a flight incident.

Owing to the poor military situation, with British Commonwealth forces on the brink of capturing Addis Ababa, the Duke of Aosta, the Viceroy of Italian East Africa, ordered the remaining three S.73s to be evacuated. After several days of preparation, they took off from Addis Ababa on 3 April 1941 with 36 men. It was planned to fly to Kufra in Libya, 2,500 km (1,550 mi) away. The S.73 had a normal range of 1,000 km (620 mi) with a 1,500 kg (3,310 lb) payload, so more fuel was added by placing additional tanks in the fuselage. All three aircraft force-landed in the desert. They were able to take off again and reach Jeddah, where they made an intermediate refuelling.

After several days of difficulties, including sand storms that clogged up the air filters, they took off again. Initially, it was planned to make another landing in Beirut but in the meantime Erwin Rommel had conquered Benghazi, so this was the final destination of the three aircraft. Two aircraft, after 10 hours of flying and the men inside almost killed by fumes of the fuel auxiliary tanks (and absolutely forbidden to smoke), they finally landed at Benghazi, after 4,500 km (2,800 mi) and over a month of travel.

Seven Belgian S.73s were flown to the United Kingdom in May 1940 and were pressed into service by the Royal Air Force. The RAF examples were used in North Africa where four where later used by the Regia Aeronautica. Some Italian aircraft were pressed into military service in East Africa while those still in Italy were used to equip 605 and 606 Squadriglie. Four S.73s survived until the 1943 armistice, three being used by the Allies and one by the pro-Axis government; all had been taken out of service by the end of the war.

Accidents and incidents

On 11 December 1935, S.73 OO-AGN of SABENA crashed at Tatsfield, Surrey, United Kingdom with the loss of all eleven on board.
On 30 April 1938 a Savoia-Marchetti S.73 of Ala Littoria crashed on a flight from Tirana to Rome. The aircraft struck the mountains near Maranola and all nineteen occupants were killed.

Operators

Civil Operators

Belgium

SABENA

Czechoslovakia

CSA

Italy

Ala Littoria
Avio Linee Italiane

Military Operators

Belgium

Belgian Air Force

Italy

Regia Aeronautica

United Kingdom

Royal Air Force
No. 24 Squadron RAF
No. 117 Squadron RAF

Specifications (S.73)

Data from World Encyclopedia of Civil Aircraft

General characteristics

Crew: 4-5
Capacity: 18 passengers
Length: 17.45 m (57 ft 3 in)
Wingspan: 24.0 m (78 ft 9 in)
Height: 4.95 m (15 ft 1 in)
Loaded weight: 10,430 kg (22,933 lb)
Powerplant: 3 x— Piaggio Stella X.RC 9 air cooled radial engine, 522 kW (700 hp) each

Performance

Maximum speed: 350 km/h (217 mph)
Cruise speed: 280 km/h (174 mph)
Range: 1,020 km (633 mi)
Service ceiling: 7,400 m (24,278 ft)

Lembo, Daniele,SIAI 81 Pipistrello, Aerei nella Storia, n.33.
The Illustrated Encyclopedia of Aircraft (Part Work 1982-1985). Orbis Publishing.

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Source: WikiPedia