Directional Gyro: Don’t Set It Here

Directional GyroHow many of us have been told to always be sure and set the directional gyro (DG) when we are in position on the take off runway? Why there? Well, the instructor would say, because we are now on a known heading and therefore the DG will be accurate right after takeoff. This procedure worked fine in the days before airplanes had engine driven vacuum pumps and airports were not so busy. But today, in my opinion, it is not just a bad place to set the DG, it is the worst place.

First of all, sitting on the runway with your back to traffic is not a good spot to be in both from a safety standpoint and from an airport efficiency point of view. We should do all we can to minimize our time on the runway. Secondly, the runway is not actually a known heading. All we know about runway 27 is that it is closer to 270 degrees than it is to 260 or 280. Not all runways are aligned exactly to the corresponding number. Then there is the problem of lining the aircraft up exactly parallel to the centerline. In addition, rushing to do this step while on the runway provides another opportunity to make an error. So you can see that setting the DG while on the runway exposes the pilot to the added risk of sitting with your back to traffic and is not necessarily the most accurate procedure.

In my view, a much better practice is to set the DG during the run up phase of flight. By that time the vacuum pressure has the gyros up to speed and since we are parked, the magnetic compass has had a chance to settle down to a good heading. Further, doing this step when we are parked allows us to take all the time we need to set the DG to the exact heading noted on the magnetic compass, thereby reducing the chance for errors. Of course it is a good practice to do a gross error check as you line up on the runway to confirm that you are taking off on the correct runway.

So how did this practice of setting the DG on the runway get to be so common? Back in the day, when most airplanes did not have an engine driven vacuum pump, they had a gadget called a venturi tube. This is a device that looks like a miniature air horn from a truck mounted on the side of the fuselage. It produced the needed vacuum to run the DG and artificial horizon by utilizing airflow through it. Therefore you did not get adequate vacuum pressure until after the aircraft built up airspeed. So, the idea was to set the DG to the runway heading as that was likely the heading the aircraft would be on when the DG became useful.

Fortunately now we have engine driven vacuum pumps and can get our gyros up to speed well before takeoff. So let’s drop the old habits, stop tying up the runways and get that DG set accurately on the taxi way.