Friday, February 19, 2010

Reverse Sensing? No Such Thing

I often hear comments from pilots about confusion over normal sensing and reverse sensing. To some it seems no big deal, but to others it is a source of worry and confusion. I am going to make what may seem to be a rather startling comment to some of you.

If you truly understand VOR navigation, there is no such thing as reverse sensing.

Okay, bear with me. The confusion arises because the majority of pilots have been taught to “fly towards the needle” when learning VOR navigation. That in turn gets interpreted to mean fly either right or left in order to get on the desired course. And therein lies the problem and the source of the confusion.

You do not fly towards the needle. Instead you fly towards a heading, and that may mean fly either right or left. It depends on where the nose of the aircraft is pointing.

In the above figure, the OBS is set to 360, same direction as the nose of the aircraft. So students have been taught to fly towards the needle, or to the left. But instead of right or left, think heading. Looking at the VOR display, a heading of 270° will take you directly to the selected course. In this case that does happen to be a turn towards the left. But now let’s spin the aircraft around, so it is flying south.

Notice the VOR display has not changed. The VOR head (as the VOR display is technically called) neither knows nor cares where the nose of the aircraft is pointed. The CDI is still deflected to the left. It is still saying that to get directly to the selected course, you need to fly a heading of 270 degrees. But now look at the DG (or heading indicator). A heading of 270° is towards your right – in the opposite direction of the needle. This, then, is the so-called reverse sensing. But if you think heading rather than right or left, there is no reverse sensing. Instead there is just a heading to fly, and that heading may be to your left or to your right.

This works for localizer as well as VOR courses. Let’s take a look at the ILS-18 into Lebanon, NH (KLEB). Here is what the instruments look like as the pilot is being vectored for the ILS, flying a heading of north. Look at the approach plate first and see where the aircraft is.

Now let’s look at the aircraft instruments. Note the aircraft is flying north, on the east side of the localizer. The VOR display says the pilot needs to fly towards the west to get to the localizer course. The DG (heading indicator) says this means a turn towards the left. Think heading, not right or left!

Now let’s look at tracking inbound on the localizer. Again, first look at the approach plate to see where the aircraft is.

Now let’s look at the instruments. Note that the aircraft is slightly west of the localizer course. The instruments show exactly the same thing – the aircraft is slightly west of course. Also note in both cases, the OBS for the VOR display is set to the final approach course. If you set the OBS to the final approach course, it results in less knob twisting, and in a busy cockpit, this is an advantage.

So you see that if you disavow yourself of the “fly right or left or towards or away from the needle” way of thinking, the entire concept of reverse sensing simply disappears.

I’ve shown it here for a localizer, since this notion of reverse sensing is very common when either flying a back course or tracking outbound on a localizer to do a procedure turn. But it works exactly the same way for tracking on a VOR approach.

Let’s look at the VOR-25/LEB. Below is a portion of the approach plate, showing the position of the aircraft being vectored for the approach.

Now let’s look at the instruments. The aircraft is being vectored on the east side of the final approach course. The OBS is set to the final approach course. The needle is deflected to the right and a heading of 330° would take us directly to the final approach course. But 330°is to our left, away from the needle. Think heading, not right or left.

Now let’s look at the picture as the aircraft has been given a heading that provides an intercept for the final approach course. First look at the approach and see where the aircraft is.

The aircraft is on a heading of 280°, just about ready to join the final approach course. This is what the instruments look like.

If you think heading rather than right or left, you can set the OBS to the final approach course, even if you are flying outbound to do a procedure turn. By the way, with an HSI you always set the course to the final approach course, sometimes called the front course, even if you are flying a Localizer-Back-Course approach.

If you are used to always setting the OBS to your heading, as students have been traditionally taught, then continue with the procedure if you are comfortable with it. However you decide to do it, though, think heading – not right or left.

I close this discussion with the following acronym.


I have used it for a long time to help students remember what radial they are on.

I = inbound or “TO” flag on VOR head

B = radial is at the bottom of the VOR head

O = outbound or “FROM” flag on the VOR head

T = radial is at the top of the VOR head

So think “inbound bottom” and “outbound top.”

Try out some of the ideas I’ve presented here yourself. You will find out they work, and they can significantly reduce the amount of confusion that occurs when doing VOR navigation.

Sunday, February 07, 2010

True, False or Maybe

Instrument flying is chock full of rules and regulations. However it is also replete with gray areas, where a clear definitive answer is not spelled out. These areas are always good to spark a lively debate among pilots. Instructors are no different – we have our ideas too about the gray areas. So let’s look at a few things that are not clearly spelled out and are sure to provoke some debate.

Question 1:

Scenario: you are flying towards the IAF for the NDB or GPS RWY 34 at Cambridge (KCBG). You have been cleared for the full approach, your direction of flight is northwest and your altitude is 4000’ MSL.

After passing over the IAF, you must get established on the outbound leg with a heading of 170° in order to do a procedure turn. Which is the true statement?

· You must make a right-hand turn to get established on the outbound course because that is the protected side shown for the hold.

· It doesn’t make any difference whether you turn right or left because you are at MVA and not in any danger of colliding with an obstacle.

Question 2:

For the same approach, for various reasons you are at an altitude of 8,000’ and need to lose a lot of altitude in order to fly this approach. ATC tells you to hold south of the NDB IAF as published and descend in the hold until you reach 3,000’.

You do as directed and reach 3000’ as you are approaching the IAF on the inbound leg of the hold. ATC clears you for the full approach.

True or false:

· You must intercept the outbound leg in order to fly out and do a procedure turn.

· You can simply extend the outbound leg of the hold for two minutes or so and then turn inbound in the elongated holding pattern.

· Both procedures are acceptable.

Bonus question:

· What is DUMDY? (Hint – it is not shorthand for Descend Undercarriage Mixture Descend some more and Yippee there’s a runway in sight)

Question 3:

Now let’s look at another approach, this one is the GPS-28 approach into Maple Lake (KMGG). Scenario: you are approaching NAZMY from the east. ATC has cleared you for the approach.

You are pretty much lined up with the final approach course but the approach plate doesn’t specify NoPT coming from this direction and the 1-minute hold depicted at NAZMY is clearly meant to serve as a procedure turn.

True or false:

· You must do a turn around holding because NoPT is not specified

· You don’t have to do a turn around holding because you are more or less lined up with the final approach course

Question 4:

You are flying the full VOR-A approach into KMIC (see IAP below). You have completed your outbound leg on the procedure turn and are ready to do a 180° turn to a heading of 121° to intercept the inbound course.

You happen to glance at your DME and it reads 9 nm. The profile view clearly states the procedure turn must be done inside of 10 nm. Which of these statements is true?

· You must turn right because that is how the procedure turn is depicted on the approach plate.

· It makes no difference which way you turn, so make a left-hand turn to 121° in order to stay within the 10 nm range for the procedure turn.

Second bonus question:

You are flying single-pilot IFR and are trying to get the weather at your destination. But just about the time the ATIS report is giving the winds, ATC starts talking again so you turn down the volume on the ATIS frequency in order to hear the controller. After several attempts to get the weather, you still don’t have the pertinent information and are getting closer and closer to your destination.

What is your best course of action?

· You never leave the ATC frequency, so just hang in there and keep trying. Maybe you’ll get lucky.

· Ask ATC for permission to leave the frequency to get the weather at your destination


As I said at the start, these are areas that can provoke a lot of discussion among pilots, so here is my two cents worth.

Question 1:

It makes no difference which way you turn in order to get established outbound.

Question 2:

Both procedures are acceptable.

First bonus question:

DUMDY is a procedural waypoint for an IFR GPS. An IFR GPS starts scaling down to 0.3 nm sensitivity two miles before reaching the FAF. However this approach does not have a FAF; you simply start to descend when established inbound. DUMDY serves as a virtual FAF for an IFR GPS. Two miles before reaching DUMDY the GPS starts scaling down to 0.3 nm sensitivity. And as a sidebar, if flying this as a GPS approach, you had better not start your procedure turn until you are south of DUMDY.

Question 3:

The second answer is correct. ATC is not expecting you to do a procedure turn. In the AIM, this is clarified, and controllers will often clear you straight in for the approach. If there is any doubt in your mind, however, query the controller about it.

Question 4:

The second one is preferred. It makes no difference which way you turn, and a left-hand turn will more likely keep you within the 10 nm limit.

Second bonus question:

The second course of action is preferable. You won’t read this in any book, but I have had both controllers and DPEs tell me they would prefer pilots ask for a frequency change to get the weather. Otherwise you are trying to listen to two frequencies at once and risk missing a call or not getting the weather (or both).

In the future I’ll try to delve into some of issues and questions like this. Your suggestions for future topics are welcome.