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Partial Panel NDB Approach
(The pictures associated with this article are being redrawn)
Ah yes, the dreaded partial panel NDB approach. Only a few years ago this procedure was required on all initial instrument rating checkrides. This is no longer the case. The current Instrument Airplane Practical Test Standard only requires a partial panel non-precision instrument approach. I have found however, that many instructors still concentrate on the NDB when training applicants in partial panel procedures. Though some think it archaic, the partial panel NDB approach will improve the pilot’s situational awareness and instrument flying ability. Long ago I inherited a student who had failed his instrument checkride twice on this procedure. In those days you had to wait 30 days to recheck after a second failure, so this lucky person and I had a whole month to explore the finer points of the partial panel NDB approach. Here are some techniques that have helped me in my work as an instructor.
On-Field NDB Approach.
First let’s get back to basics. 1. While partial panel, the turn coordinator should be used to determine straight and level flight. 2. The magnetic compass should be cross-checked to ensure that straight and level flight is occurring on the desired heading. 3. Use timed turns for small corrections of heading. Just count one second for every 3 degrees. A 3 second turn is about 10 degrees, 5 seconds for 15 degrees, etc. 4. The VSI and ALT should be crosschecked to ensure that flight is in fact level. This basic skill is not that difficult and should be mastered along with climbs, descents, and turns. The TC is the key to flying straight, however, and this is often under-emphasized.
The fundamental elements of this procedure are merely tracking and intercepting.
Here are some basic rules that should be followed during this procedure.
1. Always parallel the bearing prior to any turns for interception. 2. When intercepting a bearing, the head of the needle always falls, and the tail always rises. 3. If ever in doubt about the aircraft position left or right relative to the desired bearing, parallel the bearing and observe the big end of the needle deflecting towards the course, then correct in that direction.
1. Set up for the approach early. Tune, identify, and monitor the NDB. Review and brief the approach. Track to the station and learn through headings and groundspeed the wind direction and speed to the degree possible with equipment on board. 2. After station passage, turn to the outbound heading to parallel the outbound bearing specified in the procedure. Use a compass turn to accomplish this. Compass turns are preferable to timed turns except for small corrective turns of less than 15 degrees. 3. Whenever you are parallel to a bearing, inbound or outbound, the pointer on the ADF points left or right toward the course. One way to accomplish turns from now on in the procedure is to use the relative angle of the ADF needle to judge heading change while in a turn. For example, if parallel to a 360 outbound bearing and flying heading 360, the needle looks like this:
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1. Turn towards the head of the needle 30 degrees. To turn the aircraft to a heading of 330 in order to intercept the 360 outbound bearing, turn left (the direction the big pointy end of the ADF needle is indicating) until the deflection of the needle has increased 30 degrees. This is simple and precise.
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The turn has caused the needle to deflect an additional 30 degrees. Try making this turn with the needle as your reference, i.e. roll out from the turn when the needle has changed position by 30 degrees. Then check the compass heading to confirm, and rotate the ADF card to match the compass heading. This technique works best with ADF indicators that do not lag significantly while turning. I recommend always turning the compass card on the ADF to the heading the airplane is flying. It is a slaved card; you are the slave. It should always match compass heading just like a slaved card does.
1. When the tail of the needle has risen to the outbound bearing (remember, the head falls and the tail rises), turn to the outbound heading.
1. This turn may be easily accomplished by turning in the appropriate direction until the head of the needle is pointing aft. Confirm again with the compass.
1. Fly outbound for two or three minutes total time after crossing the NDB station depending on wind conditions. 2. Turn 45 degrees in the direction of the procedure turn. Once again the most accurate way to make this is to turn in the correct direction until a 45 degree deflection exists on the ADF. Scan the magnetic compass to confirm correct heading. Fly 1 minute.
1. In a no wind situation the 45 degree deflection should have fallen to 30 degrees or so.
1. To accomplish the turn inbound, which will be 180 degree turn to the left, turn left until the needle has changed direction 180 degrees. This will be much more accurate than a timed turn. Back up this turn with the compass.
1. Scan the TC. Upon completion of the 180 degree turn, the pilots’ inner ear will be lying to him/her. If you don’t scan the TC aggressively at this point, the turn will continue and errors will result. 2. Check compass heading, set the ADF card, and fly until the head of the needle points to the inbound bearing, in this case 180 degrees. If things are working out well this should equal a 45 degree deflection. Remember the rule. “The head always falls, the tail always rises”
1. Turn to intercept the inbound course by turning left until the needle is straight with the pointer in the 12 o’clock position. Back this up with the magnetic compass
1. Now you are established inbound and the descent to MDA can begin. Remember all course corrections from here on are nothing more than interception procedures. If, while flying inbound on the correct heading you find the needle deflecting, turn towards the head of the needle until the needle is on the opposite side of the 12 O’clock position from it’s original deflection. Reset the ADF card and fly that heading until the head falls to the correct bearing. Then re-intercept using wind correction procedures.
2. If this happens:
1. Do this:
1. Until this happens and then turn to re-intercept.
1. Remember to stay on course and to descend at 800 feet per minute.
Notes for the off field NDB approach.
1. The procedure turn and intermediate segment are the same as the on field approach and all the techniques outlined above will work equally well. 2. Once you cross the NDB and are now descending inbound stability is the key. Having crossed the NDB we should now fly the course heading precisely by using the TC and magnetic compass, and paying less attention initially to the ADF needle. When we are right over the station errors and extreme sensitivity make the ADF needle a poor tool. Fly the heading outbound however and the needle will swing nicely into position. What about drifting off course due to wind drift? Two points: a) Most pilots will fly a more accurate approach and one that meets PTS standards by ignoring the wind and flying an accurate heading. Most errors at this point occur as the pilot over corrects NDB inbound trying desperately to get that needle in the right position. Once excessive heading corrections have been made, the errors tend to compound and we are looking at a bad approach or maybe total disorientation. So the errors resulting from course corrections inbound are usually greater than the wind drift problems. b) OK so maybe there is a significant crosswind on the inbound leg.
These techniques really do work. They train the pilot by providing well defined, step by step procedures that ensure success when practiced and followed carefully. This training also seems to do much for situational awareness in general. NDB approaches may be going away, but while they are still here, and while we are flying airplanes with no GPS installed, why not train this procedure in a way that really gets results. Try it with your instrument students and let me know how you make out.
As always during flight training it is imperative that you stop instructing at some point and evaluate the work that has been done in the cockpit. When your student can do this procedure with no coaching, you will both have confidence in his/her ability to pass a checkride and fly safely in the clouds.
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