Reynolds Metals Co. reported yesterday that the April-june period this year was the most profitable quarter ever for Reynolds, the country's second largest aluminium producer.

Allegheny Airlines also reported increased earnings from its expanding domestic air operations.

Reynolds earned $59.4 million ($3.12 a share) in the second quarter, up 34 percent from $44.2 million ($2.31) in the same period last year. Sales rose to $874 million from $715 million at Reynolds, the largest industrial firm based in Virginia.

In the first six months of 1979, Reynolds earned a record $98 million ($5.12 a share) compared with $33 million ($1.68) in the 1978 period, as sales rose to $1.65 billion from $1.32 billion. Included in the six-month earnings were foreign currency translation and exchange losses of $3.5 million this year and $30.7 million in 1978.

Chairman David Reynolds said second-quarter shipments were 353,700 tons conpared with 323,000 a year earlier and 334,400 tons in the first quarter of 1979. New orders continue at high levels, and production schedules remain extended "for a substantial portion of our busineess," Reynolds said. Coupled with customer inventories at "below-average historic levels," these factors are "very encouraging as we enter the final half of the year," he added.

Allegheny Airlines, based at National Airport, reported second-quarter profits earnings of $17.2 million ($1.55 a share) compared with operating earnings of $17.2 million ($2.77) last year, as revenues jumped to $186 million from $149 million. In the 1978 period, a one-time gain from use of loss carryovers added $4 million to earnings for final net income of $21.2 million ($1.42).

For the first six months of 1979, the Washington-based carrier earned $11.6 million (92 cents) compared with operating profits of $10.2 million ($1.63), and sales rose to $328 million from $267 million. Net income a year ago included a one-time gain of $5.6 million from selling planes.

Ryland Group Inc., a home building company in Columbia, reported that second-quarter profits dipped to $1.6 million (51 cents a share) from $2 million (58 cents) a year earlier although sales contracts signed rose 26 percent to $82.6 million. Moreover, six-month earnings rose 15 percent to $2.8 million (87 cents) from $2.4 million (72 cents).

Chairman James Ryan said new orders remained strong although traffic in model homes declined. Directors voted a quarterly dividend of 12 1/2 cents a share payable July 30 to owners of record July 15.