Why PQ 17's heavy escort stopped at 25 degrees east

Why was PQ 17's heavy escort ordered not to pass 25 degrees east?

Because carrier effectiveness declined while German land-based air danger increased, and a late German surface interception became progressively less likely unless intelligence showed a sortie underway.

Farther east, Arctic weather and sea conditions increasingly reduced a carrier's ability to operate its air group. Meanwhile, land-based Luftwaffe bombers could fly in worse conditions and bring stronger local air power against the escort. The capital ships' main purpose was to counter a German surface sortie, so their exposure had to be justified by the likelihood of that threat. By the turn-back point, intelligence should normally show whether German capital ships were preparing to sail. Without such indications, a successful late interception became less likely, while evidence of an impending sortie could justify continuing east. The closing comparison also identifies the He 177 as a dangerous Arctic attacker when serviceable because of its heavy armour-piercing bomb load, while acknowledging its reliability problems.