Narrative:

I was captain on the flight. It was the first officer's takeoff and as he pushed the power up for takeoff, the takeoff warning horn sounded. I called the abort, notified tower that we were aborting the takeoff and informed the tower we would not need any assistance. We pulled off the runway, accomplished all appropriate checklist items, reset and checked the speed brake handle, and made a PA, after which we taxied back out to the runway and a normal takeoff was accomplished. Once airborne, we discussed the abort. All of us agreed the speed brake was full forward, however, the jumpseat rider said he noticed the speed brake handle seemed to move down (not forward) as I pushed on the handle. It's important to understand that the speed brake in the B737 has 2 axis of motion. The first is forward and backward, the other is a spring loaded tension that moves the speed brake handle back down towards the floor when we extend it up to take it out of the detent. Sure enough, when I examined the speed brake handle and pulled it up it would bind so that it would stay in the extended position, instead of recompressing. I think most pilots think that the takeoff warning horn is triggered by the speed brake lever not being stowed or, in other words, not full forward. In fact, the takeoff warning horn is entirely a function of the speed brake compressing and pushing down on a micro-switch. In this particular case, with the speed brake handle stowed, and compressed just enough to get trapped in the detent, all indications from the pilot's perspective seemed normal, but if you look at the micro switch, it is clearly apparent that the switched remains uncompressed. As an experiment, when we landed, I asked the first officer to leave the flaps at the #1 position, reset the trim and stowed the speed brake. I observed the speed brake seated in the detent, but I could also see it partially binding and not fully compressed. When I move the throttles up to taxi, the takeoff warning sounded. At the gate, I notified dispatch (as outlined in our operations manual for aborts) who conferenced me into maintenance control, who talked with our local maintenance to work on the problem. A logbook entry was made, and signed off by maintenance. I think there may be an important lesson here. I wonder how many of our aborts are actually caused by a similar type of binding without the crew knowing. Our present system doesn't account for this in that the only place we really deal with the speed brake is in the cockpit preparation, and in the expanded checklist. It only talks about making sure it is fully forward, nothing about pushing down. In addition, on the through flts, there is no checklist item relating to the speed brake at all. When I handed the aircraft over to a new crew at the end of the day, I brought them into the cockpit to show them what had happened. Neither crew member knew that the trigger for the horn was the handle being compressed. Both said that they only pushed it forward, which is exactly what the manual tells us to do. It's also important to note that in this particular case, the handle compressed enough to hold it in the detent. Had I not specifically looked and saw that it was partially binding, I would have never known. I cannot believe that this set of circumstances were unique to this airplane. I think it's important that we educate our pilots as to what the actual trigger is for this horn. I have a suspicion that this is the cause for takeoff warning horns more times than we think. There is a lot of dust in that area, and I have a feeling these handles get dirty and bind on a regular basis. If that's the case, there might be an easy fix of simply including some type of visual check of the speed brake in our flow to see if it compressed. (Once you know where to look, it's easy to see.) or incorporating a checklist item that incorporates a flow that simply pushes down on the handle.

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Original NASA ASRS Text

Title: A B737 FLT CREW RECEIVES A TKOF WARNING HORN AND ABORTS TKOF DUE TO THE SPD BRAKE LEVER NOT BEING ALL THE WAY STOWED.

Narrative: I WAS CAPT ON THE FLT. IT WAS THE FO'S TKOF AND AS HE PUSHED THE PWR UP FOR TKOF, THE TKOF WARNING HORN SOUNDED. I CALLED THE ABORT, NOTIFIED TWR THAT WE WERE ABORTING THE TKOF AND INFORMED THE TWR WE WOULD NOT NEED ANY ASSISTANCE. WE PULLED OFF THE RWY, ACCOMPLISHED ALL APPROPRIATE CHKLIST ITEMS, RESET AND CHKED THE SPD BRAKE HANDLE, AND MADE A PA, AFTER WHICH WE TAXIED BACK OUT TO THE RWY AND A NORMAL TKOF WAS ACCOMPLISHED. ONCE AIRBORNE, WE DISCUSSED THE ABORT. ALL OF US AGREED THE SPD BRAKE WAS FULL FORWARD, HOWEVER, THE JUMPSEAT RIDER SAID HE NOTICED THE SPD BRAKE HANDLE SEEMED TO MOVE DOWN (NOT FORWARD) AS I PUSHED ON THE HANDLE. IT'S IMPORTANT TO UNDERSTAND THAT THE SPD BRAKE IN THE B737 HAS 2 AXIS OF MOTION. THE FIRST IS FORWARD AND BACKWARD, THE OTHER IS A SPRING LOADED TENSION THAT MOVES THE SPD BRAKE HANDLE BACK DOWN TOWARDS THE FLOOR WHEN WE EXTEND IT UP TO TAKE IT OUT OF THE DETENT. SURE ENOUGH, WHEN I EXAMINED THE SPD BRAKE HANDLE AND PULLED IT UP IT WOULD BIND SO THAT IT WOULD STAY IN THE EXTENDED POS, INSTEAD OF RECOMPRESSING. I THINK MOST PLTS THINK THAT THE TKOF WARNING HORN IS TRIGGERED BY THE SPD BRAKE LEVER NOT BEING STOWED OR, IN OTHER WORDS, NOT FULL FORWARD. IN FACT, THE TKOF WARNING HORN IS ENTIRELY A FUNCTION OF THE SPD BRAKE COMPRESSING AND PUSHING DOWN ON A MICRO-SWITCH. IN THIS PARTICULAR CASE, WITH THE SPD BRAKE HANDLE STOWED, AND COMPRESSED JUST ENOUGH TO GET TRAPPED IN THE DETENT, ALL INDICATIONS FROM THE PLT'S PERSPECTIVE SEEMED NORMAL, BUT IF YOU LOOK AT THE MICRO SWITCH, IT IS CLRLY APPARENT THAT THE SWITCHED REMAINS UNCOMPRESSED. AS AN EXPERIMENT, WHEN WE LANDED, I ASKED THE FO TO LEAVE THE FLAPS AT THE #1 POS, RESET THE TRIM AND STOWED THE SPD BRAKE. I OBSERVED THE SPD BRAKE SEATED IN THE DETENT, BUT I COULD ALSO SEE IT PARTIALLY BINDING AND NOT FULLY COMPRESSED. WHEN I MOVE THE THROTTLES UP TO TAXI, THE TKOF WARNING SOUNDED. AT THE GATE, I NOTIFIED DISPATCH (AS OUTLINED IN OUR OPS MANUAL FOR ABORTS) WHO CONFERENCED ME INTO MAINT CTL, WHO TALKED WITH OUR LCL MAINT TO WORK ON THE PROB. A LOGBOOK ENTRY WAS MADE, AND SIGNED OFF BY MAINT. I THINK THERE MAY BE AN IMPORTANT LESSON HERE. I WONDER HOW MANY OF OUR ABORTS ARE ACTUALLY CAUSED BY A SIMILAR TYPE OF BINDING WITHOUT THE CREW KNOWING. OUR PRESENT SYS DOESN'T ACCOUNT FOR THIS IN THAT THE ONLY PLACE WE REALLY DEAL WITH THE SPD BRAKE IS IN THE COCKPIT PREPARATION, AND IN THE EXPANDED CHKLIST. IT ONLY TALKS ABOUT MAKING SURE IT IS FULLY FORWARD, NOTHING ABOUT PUSHING DOWN. IN ADDITION, ON THE THROUGH FLTS, THERE IS NO CHKLIST ITEM RELATING TO THE SPD BRAKE AT ALL. WHEN I HANDED THE ACFT OVER TO A NEW CREW AT THE END OF THE DAY, I BROUGHT THEM INTO THE COCKPIT TO SHOW THEM WHAT HAD HAPPENED. NEITHER CREW MEMBER KNEW THAT THE TRIGGER FOR THE HORN WAS THE HANDLE BEING COMPRESSED. BOTH SAID THAT THEY ONLY PUSHED IT FORWARD, WHICH IS EXACTLY WHAT THE MANUAL TELLS US TO DO. IT'S ALSO IMPORTANT TO NOTE THAT IN THIS PARTICULAR CASE, THE HANDLE COMPRESSED ENOUGH TO HOLD IT IN THE DETENT. HAD I NOT SPECIFICALLY LOOKED AND SAW THAT IT WAS PARTIALLY BINDING, I WOULD HAVE NEVER KNOWN. I CANNOT BELIEVE THAT THIS SET OF CIRCUMSTANCES WERE UNIQUE TO THIS AIRPLANE. I THINK IT'S IMPORTANT THAT WE EDUCATE OUR PLTS AS TO WHAT THE ACTUAL TRIGGER IS FOR THIS HORN. I HAVE A SUSPICION THAT THIS IS THE CAUSE FOR TKOF WARNING HORNS MORE TIMES THAN WE THINK. THERE IS A LOT OF DUST IN THAT AREA, AND I HAVE A FEELING THESE HANDLES GET DIRTY AND BIND ON A REGULAR BASIS. IF THAT'S THE CASE, THERE MIGHT BE AN EASY FIX OF SIMPLY INCLUDING SOME TYPE OF VISUAL CHK OF THE SPD BRAKE IN OUR FLOW TO SEE IF IT COMPRESSED. (ONCE YOU KNOW WHERE TO LOOK, IT'S EASY TO SEE.) OR INCORPORATING A CHKLIST ITEM THAT INCORPORATES A FLOW THAT SIMPLY PUSHES DOWN ON THE HANDLE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.