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If the thrust reversers on an Original-series 737 (the 737-100 and -200) are 2 inches or more from the stowed position, and hydraulic pressure is removed from the reverser actuators, the reversers will travel to the fully-extended position under aerodynamic loads at airspeeds down to below 130 KEAS (knots equivalent airspeed), as was discovered during the investigation into the crash of Pacific Western Airlines Flight 314. To quote the official accident report:

... It was found that the air loads on the reversers were insufficient to deploy them further if they were freed with the leading edges 1 inch out from the closed position. If the initial deflection was increased to 2 inches, they invariably deployed fully under the air loads. At intermediate settings, deployments only occasionally occurred. ... [p. 17-18 of the report; p. 23-24 of its PDF file]

... The ground runs showed that partially opened thrust reversers deployed at various speeds during taxi tests with manually controlled airlground logic. The rate of deployment varied with the amount of the initial open condition of the reverser doors and also varied with speed. In all cases partially opened doors deployed fully before 130 Kts. [p. 18/24]

However, the 737 Originals use target-type reversers (you know, the kind with the two buckets that close behind the engine’s tailpipe, the ones that everyone immediately thinks of when they hear the phrase “thrust reverser”); I can easily see how they would be blown to the full-open position if the engine(s) aren’t operating, but, if they are running, then, without hydraulic pressure holding the reverser buckets in position, shouldn’t the engine(s)’ jet blast force the buckets apart and open a path between them for the engine efflux to exit?

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If the buckets could be opened by engine thrust, they would be ineffective to reduce speed. By the same token, you want airflow to help them deploy when needed. The controlling force then is the hydraulic actuators. Without hydraulic power it would be very difficult to achieve positive control over the buckets' position.

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  • $\begingroup$ If the buckets could be opened by engine thrust, they would still be effective at reducing speed as long as their hydraulic actuators could hold them closed against the force of the engine's airflow. $\endgroup$
    – Vikki
    Jan 29, 2019 at 3:45
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    $\begingroup$ @Sean: But if you're landing and using thrust reversers, I'd think the design assumption would be that you need them to keep working in order to stop before you run off the end of the runway, no? $\endgroup$
    – jamesqf
    Jan 29, 2019 at 4:27
  • $\begingroup$ @Sean That doesn't make any sense. If the hydraulic actuators can hold them closed, the buckets cannot be opened. If they can be opened, the actuators are useless and would be ineffective in slowing the aircraft. $\endgroup$
    – Juan Jimenez
    Jan 30, 2019 at 10:18
  • $\begingroup$ @JuanJimenez: If the hydraulic actuators can hold them closed, the buckets cannot blow open as long as the actuators continue to receive hydraulic pressure - they would still be useful for braking, but could potentially blow open if hydraulic pressure to the actuators was lost. $\endgroup$
    – Vikki
    Feb 28, 2019 at 4:01
  • $\begingroup$ No @Sean they won't be, because no redirection of thrust happens until the buckets open. $\endgroup$
    – Juan Jimenez
    Mar 1, 2019 at 9:22
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Fortunately the airflow around the engine at that phase of flight is hopefully faster than the airflow passing inside the engine, since you're in need to slow down the plane. In every other phase where you need engine thrust it would probably interfere as you asked. As well as for petal reverser, airflow keeps it wide open.

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    $\begingroup$ If the flow inside the engine is slower than the freestream, then it's producing drag, not thrust. Even at idle jet engines typically provide some amount of thrust. $\endgroup$
    – fooot
    Jan 28, 2019 at 20:19

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