Altitude above ground level indication in small planes
 

Hi,

I have a probably very stupid question that can hopefully be answered by someone....when preparing e.g. a Cessna 172 for a traffic pattern trip over a certain airfield I adjust the QNH to the correct setting so that my altitude gauge is showing the field elevation of that airport.
Lets say I want to fly my traffic pattern in an altitude of 1000feet above ground level. The airport altitude is 350 feet above sea level.
During climb do I really need to reckon 1000 feet + 350 feet to know that I must stop at 1350feet for the height of the pattern? Or is there a trick to show me the above ground altitude inside the cockpit of the cessna?

With the a320 I would just look at the radio altimeter for that.

Thanks!

Best regards Andi

Radalt no good as it will vary as the terrain beneath slopes - the ground is rarely completely flat.

So I think what you are looking for is a pressure setting which reads height above touchdown? You could use QFE then, which does exactly that. I hesitate to say this, as I know it will almost certainly degenerate into a ‘I love QFE vs QFE’s crap’ debate but it’s a thing and amazingly it works!

For the next poster along please note that I haven’t expressed a preference - I’ve used QFE and QNH for circuits with equal skill (or lack of).

ps. I can’t be arsed to be picky about definitions of height, altitude etc. I’ll leave that for someone else.

YES!

You need to know where you are and where you need to be when all the electronics fail!

Not true. In an A320 you would brief the circuit height reference to QNH and set it in the altitude window on the FCU or just remember it. Your radalt will be going up and down with every little hill or building you fly over.

You fly at whatever altitude the airport operator tells you in the conditions of use of the airport. It you're uncertain, phone the operator or the ATS unit (if there is one) and ask.

So in case of a single engine failure during takeoff roll with an A320 I would need to keep the altitudes 350feet + 1640feet (engine out altitude) = 1990 feet and 350feet + 3000 feet (altitude for start of open climb) = 3350 feet in mind before each start, since 1640 feet and 3000 feet are company specific preset values for engine out procedure?

I take it that 737Andi has never flown a circuit in a 172, much less an A320.

You are absolutely right. Im just a flight simmer trying to learn this fascinating stuff. If i said something wrong you are very welcome to correct me. Any information is highly appreciated. Thank you!

Google for altitude and height, those mean 2 very different things in aviation. In essence, your understanding seems to be correct.

So, YES. If you want to fly certain HEIGHT above airport ELEVATION, but your instrument is showing ALTITUDE - this is the overwhelmingly most common case - you need to do the math. ELEV (given) + HEIGHT (desired) = ALTITUDE (required). Each and every time, for all the different required values.

Im luckily ;) aware of the differences in local height above sea level, height above field and flying with qnh set to standard above transition level.

Im however very astonished that pilots need to do the math with eo accel altitude when being in a critical flight situation. I would have suspected an easier option for the pilots so that they know when to decrease climb angle at accelaration height to reach green dot speed. In normal flight the AP is doing that job automatically but this is not the case in one engine out abnormal procedure.

MarcK.

TbF 737Andi in a recent post describes himself as a flight simmer, so has an interest in aviation but maybe not much knowledge. Which is why he comes here for enlightenment.

In the case of an engine failure in an A320 the acceleration altitude is an altitude and therefore does not need to be corrected for the elevation of the airport. 1640 is quite a specific figure so it probably relates to a specific airport. Where did you get that number?

In addition, the acceleration altitude is a minimum. You have to be at or above that altitude and you also need to have the engine secured and have completed any initial turns in the procedure. Open climb doesn't happen at an altitude, it happens at a speed. At (or above) the acceleration altitude, you select VS 0 (push to level off), retract flaps on schedule, then as you get to green dot speed you pull for open climb and set the thrust to MCT.

In summary: For an A320 the engine out altitude is given by the take-off performance calculation and may be different for every take-off. It is an altitude and does not need the airport elevation added (take-off performance has already done that for you). You pull for open climb when you reach green dot speed, not an altitude, so you don't need to make any adjustments there either.

Hope that helps.

Andi - what would you do if you were going to fly a circuit at an airfield 1800ft above sea level?

Or even 1,700ft alt (visual circuit altitude at Farnborough)

Thank you AerocatS2A....everything is perfectly clear now :)

Luckily the designers of the airplane knew calculation inflight is for practical purposes impossible.

We calculate (read from a table) the EO ACC alt during the flightdeck preparation. Then use MCDU page TAKEOFF PERF to enter a reminder, key 5R off the top of my head. Once you lose the engine (about 10x a year in the SIM) you decrease the pitch and start accelerating at or above it. No calculations required, you just read the figure from MCDU and compare to the indication of the altimeters.