SNECMA Note No. 63/667: investigation into a DS 19 steering accident (1963)

In this document

The accident and the context of the investigation

This confidential SNECMA note (No. 63/667, dated 20 June 1963) reports on the inspection of the steering components of a DS 19 registered 160 KL 77, put into service on 3 October 1962 and with 20,900 km on the clock. The incident, which occurred on 10 May 1963, is described as follows: "The car veers to the left, difficulties in straightening the vehicle which leaves the road and hits a tree" p. 16.

The inspection took place on 30 May 1963 in the presence of Citroën representatives (Messrs. Clément, Lebreton) and SNECMA representatives (Mousseau, Picquet, Protat) p. 16.

Operation and testing

The report explains the principle of the hydraulically assisted steering, with its two distributor valves and its rotary coupling p. 16-17. Static and road tests revealed "no steering anomalies" p. 17, other than significant pitching when cornering to the left, which Mr. Clément attributes to a possible deformation of the torsion bar during the impact p. 17.

Dismantling and seizure hypothesis

Dismantling the distributor shows a slight oil leak at the rotary coupling, but the valves — mounted with a clearance of 3 to 4 μ — show no signs of seizing p. 18. Tests involving intentional blocking with a shim, contested by Mr. Clément regarding their methodological validity p. 19, show that immobilising a valve can lead to a "complete steering lock" due to the formation of a "hydraulic cushion" p. 19.

Oil analysis

The analysis reveals deposits and particles in the oil downstream of the filter (filtering capacity of 60 to 70 µ):

Conclusion

The report concludes that "the probabilities of steering lock due to the foreign bodies we found are not zero", while admitting that the tests on the crashed car did not make it possible to verify this hypothesis p. 20. A photographic appendix illustrates the four categories of particles found p. 21.

Context

The DS 19, unveiled on 6 October 1955, was distinguished by its hydropneumatic suspension designed by Paul Magès, then at the Citroën design office which he had joined in 1925 (autobiographical notes); the power steering shared the same technical philosophy of hydraulic precision with low tolerances. SNECMA, an aeronautical engine manufacturer, had a test centre at Villaroche at the time; this document illustrates the technical exchanges between manufacturers regarding an incident involving a fleet or test vehicle.

Archivist's perspective

This report is valuable because it documents, from the inside, the contradictory debates between Citroën and a client (or technical partner) faced with a steering incident — a debate that seems to continue in the handwritten notes criticising this same report and in the subsequent correspondence, notably the letter from P. Mahoudeau of 27 June 1963.

It concretely illustrates the technical sensitivity of the mechanisms with very low clearance (3 to 4 µ) designed by Magès: the slightest impurity, even without leaving a visible trace on the parts, could, according to the engineers, seriously disrupt operation. This observation probably sheds light on Magès' constant concerns regarding the cleanliness of circuits and the reliability of hydraulic distributors, themes found in his work on valves p. 18-20.