On my iPAD3 the GS is always 1/2 scale deflection down when on the Glide Slope. Surprisingly, my Chinese GPS navigator is as accurate as the actual ILS needles (bearing in mind the hide & seek factor of the GS needle in SD). However, all that is beside the point. The real point is that here all is not at the pilot's disposal. Having some approximate information is better than having zilch & a psudo ILS that does not play hide and seek would not make flying too risky. Hiding it would not enhance safety.

Very Newbytheway

DurhamTeaseAirport

One of the first things that we do as pilots when preparing to take off is set the altimeter to the regional QNH to display altitude and one of the last changes that we make to the altimeter as we approach the field is to set it to the runway threshold QFE to display height.

So why not have a feature in SD which allows for the same operations in order to correct any inaccuracies in the GPS acquired altitude as displayed on the moving map screen and any derived calculations that SD makes.

A "slider" (knob or something) on the side of the map which allows me to "adjust" the displayed altitude to that as shown on the cockpit altimeter would resolve the problem and improve the accuracy of any subsequent height based calculations such as the virtual radar, RoD and glide slope display on the ILS.

I have to say that the difference between the GPS SD displayed altitude & the cockpit altimeter has been, in my experience, very little and I have been really pleased with the accuracy of the iPad derived GPS data however I do accept that this is solely dependent on the visible number and orbital position of the GPS satellites.

Having said this it goes without saying that the primary source for altitude/height remains the cockpit altimeter.

One final thought that needs clarification which sprucegoose's post correctly identifies. The QFE measures height above the airport's chosen runway however, as far as I can make out, the height as indicated by SD uses terrain clearance at the current GPS coordinates, whereas the altitude readout gives AMSL, which means I would never use SD's height option when approaching an airport. I stick to the altitude option on the display otherwise sprucegoose's noted QFE/height inaccuracies will always be present and the height information unreliable at any given point if you treat it as the QFE readout while carrying out an approach.

Biggles -- EGKB

Amen, Biggles.

You could just forget about quaint old QFE like everyone else in the world does

Hello Steve. Hope you are well.

An easy way to calcul;ate your rate of descent is the 5:1 rule

Say you are at 95 knots on the glideslope your rate of descent should be 95x5 which is 475 ft per min

100 knots=500 ft per min and so on

Simples!

Regards

Paul

Please note page 1 where the speedx5 rule idiosyncracies were referred to on the 21.8.2012 by Biggles-EGKB. Approximate deductions,lets face it, are not as accurate as a GPS derived constantly updated instantly displayed values.

Very Newbytheway

DurhamTeaseAirport

Thanks runways. Point well made.

It's all about accurate information rather than "guesstimate" through a rule of thumb. The rule of thumb only works if you're in the right place in a 3D environment.

In my experience life isn't like that, and flying certainly isn't!!!

Also you're asking to multiple 5 (GUESTIMATE 1) by a piece of information that the normal cockpit doesn't have namely GROUNDSPEED. So shall we have another "guesstimate" of that (GUESTIMATE 2).

I've always wondered what "GUESTIMATE 1" x "GUESTIMATE 2" gives you. Neither my Pooleys nor my calculator can work it out. Perhaps its another mown hedge at the upwind end of the runway!!



Biggles -- EGKB

Yes.

Just for the sake of reference:

The exact formula for a 3° descent rate is:

GS(kts) x 5.3 = rate of descent (fpm).

If you use the value of 5 instead of 5.3 you will get a 2.8 degree approach angle.

Greetings, -- Chris.