The Lockheed JetStar had a excellent speed brake system and we used it almost all the time on descent. In the picture below you can see the speed brake extended while on the ground, it is located on the lower fuselage aft of the landing gear.
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Photo © Johan Ljungdahl
Extending it on the ground also allows entry to the aft compartment commonly known amongst JetStar operators as the boiler room or hell hole for maintenance and preflights.
On the P&W powered JetStar’s to prevent compressor stalling below 80 percent RPM air is automatically bled off the 4th stage of compression by opening a bleed strap, so basically the engine does not produce much power under 80%. By bleeding this air off there is not enough bleed air that can be taken off at the 9th stage, which is used for pressurization. So on descent in order to maintain cabin pressurization one engine, usually #2 engine must be kept above 80 percent and this one engine will provide enough forward thrust that it inhibits speed reduction, especially when slowing down to meet the 250 knots under 10,000 feet rule. If we had to stop at some altitudes on the way down, we would just bring another engine, #3 up just above 80 % and leave the speed brake extended.
We usually extended the speed brake in the mid to upper 20’s on descent to start slowing the airplane down. There is hardly any pitch adjustment needed and while there is a slight noise, there is no rumbling like wing mounted speed brakes. We usually kept the speed brake out until we reached 200 knots, the maximum speed for approach flaps and retracted the speed brake while the approach flaps were coming out. There are only 2 positions, extend and retract, but the switch had a center position that just removed the hydraulic pressure and it allowed to speed brake to be pushed up somewhat by the airflow, but it still added some drag.
If the speed brake is not retracted when full flaps is extended, then we would get an aural warning and a warning light, both not cancelable. If for some reason the speed brake did not retract because of a mechanical problem there would be a lot of metal left on the runway on landing, and it has happened.
To prevent accidental speed brake retraction on the ground in case there is someone standing on the brake, which had built in steps while accessing the boiler room, there is a ground safety valve in the retract hydraulic line that prevents hydraulic pressure from retracting the speed brake, it does retract fairly fast. On the real early JetStars this valve was located in a small panel on the fuselage just inboard of the number 2 engine. When extended on the ground the bottom of the speed brake was only inches from the ground and had to be raised for towing so all JetStar operators had a support cable that lifted the speed brake high enough to allow towing. In one case I heard, someone pushed the door up by hand and into the door lock while the safety valve was closed for towing out to the ramp, the airplane then took off and when the speed brake was extended, it operated properly but would not retract and they had to land with the speed brake extended. (lots of metal on the runway), I think is was a military JetStar.
Lockheed on later models and issued a service bulletin to modify the earlier ones, relocated the valve to the sill of the speed brake housing and if this way if someone pushed the speed brake up it would trip the safety valve back to its normal position.