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I need help with the NAV button

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Hello pilots! As you may notice, I need help with the NAV button on the Auto Pilot. What is it? What is it for? How can I use it? I read the Auto Pilot help in the Learning Center, but I still don't know what is that button for and how can I use it. Any help?

Another question off topic: If I'm in FL330, what could be the speed of the aircraft? How can I know the speed in which I should travel while I'm in different cruising altitudes?

Thank you, and have a nice flight! 🙂

10 Responses

PH Guest

If it is the switch where you have a choice between GPS or NAV then under NAV your autopilot will follow/tune VOR's as selected via the FMS or NAV1/2 under GPS it will guide you along the preselected GPS course or flight plan. What a/c?
In the "book" for the a/c you use it should indicate cruise speed as a mach no. In the real world this can be dependant on if you are cruising for best economy, fastest etc.

Pro Member Chief Captain
Alex (Fire_Emblem_Master) Chief Captain

Ok, here's your answer.

the NAV button is related to the GPS/NAV switch on the autopilot. When you click it to GPS, and hit the NAV button, it will follow the course line on the GPS.

It can also be used to have the autopilot fly the ILS approach for you.

The cruise speed differs depending on the aircraft, but you can see your ground speed by going into the GPS, and looking to the lower left corner of the GPS screen, it'll say G/S and then a # below that.

first of all, it depends on which aircraft you are flying at FL330. Also, the general cruise speed is not entirely determined by altitude. for example

You take you 757-200, with a cruise speed of Mach .82, and fly it at FL240. Since the air is more dense at FL240 than it is at FL330, you're going to have to fly at a higher Indicated AirSpeed in order to get the Mach number you're looking for, OR, you may not get it at all, and you may have to fly too fast, and overspeed before you hit the Mach number.

To simplify, most jet aircraft cruise at a Mach number, be it .72 or .85. However, you cannot attain Mach .85 at FL 240, because the air is so dense, the aircaft can not fly that fast without seriously damaging the structure, or, having full throttle, and burning tons of fuel.

Also, have you ever looked at the Fuel Flow indicators on your aircraft. You'll notice, that, in a 757-200, 82% N1 Gets a fuel flow of Approx. 8,700PPH per engine on the ground, but cruising at FL310 with same 82% N1, gets a fuel flow of about half that. That's because, like i said, the air is thinner, the easier it is to move the aircraft through it, and the less fuel is required to accelerate that air with a force powerful enough to move the aircraft at the same speed. You'll also note that the thrust actually goes DOWN the higher you climb, cause, again, the air is thinner, and there's less of it shooting out of the engines.

if u need more help, let me know, either here or MSN

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Thank you, FEM for your answer. Since I can't find you on MSN right now, I will post my answer with a few comments here.

1) When I switch to GPS in the NAV/GPS button, and then click on NAV, will it work if I have the HDG button pressed? Or do I have to click on HDG again to deactivate it and then press NAV?

2) I understood the part of the cruising altitude and speed but the problem is that I can't go that fast when flying high. Most of the time, I use my A319 or A320 to complete my flights of my VA. I also use other aircraft... Ok, the problem is that when I'm flying at FL330 or more (maybe less too) my speed is 270 or 280. When I turn, my speed goes up and I go overspeed. I'm not descending while turning. So, what happens is that there's too much pressure for the aircraft and the flight is over. When I overspeed I try to slow down, but with no success...too much pressure for the aircraft. I don't have to overspeed, and my solution is going at 250 knots. That's slow (I think) when I'm at FL 330 or more.
What can you tell me about that? Any help?

Pro Member Chief Captain
Alex (Fire_Emblem_Master) Chief Captain

The HDG button SHOULD go off, but if not, turn it off.

Ok, again, you have to look at the Mach Number, not airspeed. the 757's indicated airspeed at FL330 is somewhere around 280-300KIAS, and that keeps at a mach of .82. Now, I dont know about the airbus, so you may be flying too high for it, if it says it cruises at FL330, then you have to find out it's cruise, in Mach. Then, go into your autopilot, and stick that number in there. Once you reach FL330, turn on Autothrottle, and hit the MACH button, and you should accelerate to and cruise at that speed.

PH Guest

FL330 is fine for the "bus"....have been up to 380 in the past when extremely light. Speed- best bet unless you have a POH is to fly up to the barbers pole, on the Airbus this is the red line on the speed tape (left side of the AI) convert this then to mach no. from IAS. On the scarebus take a look at the ND CRT next to the AI as you will be able to see the Groundspeed readout which in nil wind will read about 450-500kts at a guess. Are you using the autopilot on the Airbus as the only reason that your IAS/Mach no will increase in a turn is if you are losing altitude. This should not happen with the AP/AT engaged.

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Well, PH, it happens, but not necessarily. I am not losing altitude, but the speed increases when I turn. That never happened to me.
Thanks for your help, anyway.

FEM, I looked at my Mach and it tells me .81. When I turn (and sometimes going straight) the Mach speed decreases 😳 and then increase again. That's crazy. Maybe the Airbuses have little problems. That's a shame... 😞 Well, maybe I'm misundertood! Maybe I'm wrong. Who knows? Me? Well, I can't tell. That never happened to me. I will try with my favorite 777-300.

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Alex (Fire_Emblem_Master) Chief Captain

Wow. Perhaps you have a bugged 'bus. It's ok to fly near the overspeed, but not too close, try to stay behind it by 10 KIAS at least. Also, shoot for a cruise of Mach .78-79. I dont think that aircraft is made to fly that fast. Also, watch your fuel flow, if it's way high, its definately not meant to fly that fast. If you really accelerate in a turn, use the spoilers, they'll slow you down.

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Ok, FEM, thank you very much! I'll give it a try tomorrow and see what happens.

PH Guest

Mach .82 is target speed for the Airbus 320

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Yeah, well, I mostly fly at .81 but that's too close.
Where did you get that information, PH? I found a website in my VA that has information about Flight Levels, Cruise Level, etc.

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