BFL and why it matters

BFLFor those pilots out there this post will cause you to wonder “why is he writing that?” followed by: “I know about that.”

I know you do – this isn’t for you.  It’s for everyone else.  With the high profile accident at Bedford, MA (KBED) this past weekend with a Gulfstream G-IV N121JM running off the departure end of its runway, it at least warranted a post.  While the cause of the accident is yet to be determined, a lot of our non-flying followers and members queried us about what happened.  While we can’t answer, we thought it at least appropriate to have a balanced field length or BFL explanation since when everything works as advertised, balanced field length is what makes every departure, or aborted take off go smoothly.

The good news is that BFL is all about safety.  And while a non-pilot will never have to compute it, knowing what it is will at least make you feel better about those hot and heavy take offs on days when you think that the airplane is working extra hard. 

Balanced field length (BFL) is a number that is critical to high performance aircraft operations. Though not widely discussed with those who sit in back of the aircraft, it is a number that makes all take offs from different size runways ones that end safely in the event of an engine failure. A non-technical and simplified definition can be summarized as when two distances equal each other – the one you need to abort a take off and the one you need to complete it on one engine safely, or put another way:

Is your runway long enough to stop the aircraft below the go / no go, or “decision speed” (known as V1) and not go off the runway while also being long enough to continue the take off if the engine failure occurs after V1?

If those two distances equal each other, then the aircraft can operate safely from that runway under specific load, wind, and temperature conditions.  Another important observation is that when there is *plenty* of runway to stop (because, let’s say, you are lighter than normal), you might experience a reduced thrust take-off: In this instance the pilot simply takes longer to accelerate (less power set) and let’s the aircraft get to the speeds they need, but not without pouring gobs of fuel into the fire.

The reality of operations in any aircraft is that the crew is actually constantly making compromises to make this number work – and they are doing this in the interests of safety.  When a crew doesn’t have to make a compromise, then the airplane is performing at its maximum limits (in terms of weight, outside air temperature and density altitude) and leaving with *everything* – i.e. all the people it can fit and all the fuel it can hold.

But such is not the typical day of the typical jet throughout the big boy and smaller airports of the world. To be safe, fuel loads may be reduced and passenger loads are limited in order to make each take off within the tolerances outlined by the law and the manufacturer.

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