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Randh

Question about minimum speeds on approach

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Hi, all,

 

First off, I'm a huge fan of the PMDG 747. I really wish all payware addons are this well made, and having also bought the 737 with equal satisfaction, I know PMDG is a fantastic company who puts a lot of thought and effort into making a quality product.

 

On two separate occasions, I found myself stalling into terrain on approach to my destination. The first time was enroute to JFK. I was doing the Belmont Visual and it happened when I was on the final left turn before 22L. When that happened, I figured I must have been at a speed that was safe for level flight, but got into a dangerous angle of attack when banking left, and slapped myself on the wrist for such a terrible and "deadly" mistake.

 

That was a year ago, and just now, I stalled upon approach again. This time, however, was a little different. I was enroute to LAX and found myself having to maintain separation with a landing aircraft ahead. So, with flaps on 25, I slowed to about 120 IAS. According to the tape, this was about 10 knots above the minimum. Once I reached about 123 knots or so, I stalled and entered an uncontrollable dive into terrain. On this approach, my wings were level, I was at a normal pitch for approach, and winds were calm.

 

I figure I must've had too much trust in the speed tape, and should have maintained a greater margin above the red bar. That said, is there something else that might have happened? Perhaps I had entered an incorrect gross weight into the CDU or there was something that wasn't calibrated correctly which made the aircraft provide inaccurate information? Or was my mistake just that I should have given myself a little more margin? I would think that when it comes to instruments, it would try to "overestimate" such that it would have a red bar 10 knots ABOVE the absolute minimum.

 

I was able to reproduce the same scenario a few times in a controlled test, and each time it seemed to be the same: The tape said my minimum speed was 110 knots, but once I reached just above 120 knots, it entered a stall and uncontrolled dive. The angle of attack didn't seem to make much difference, whether my pitch was level, up, or down, the same thing seemed to happen.

 

I'll appreciate any advice. Thanks!

 

Randall Huck

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Hi,

 

When you say minimum speed, are you talking of the top of the yellow or red tape on the speed tape of the PFD?

 

If you stay above the yellow limit, you shouldn't not be stalling unless:

_ your weight (GW or ZFW) entered in the FMC is wrong and lower than the real weight of the aircraft, hence wrong limits calculated

_ or you do abrupt manoeuvres exceeding the 1.3g margin.

 

In the first case, it could be a problem of units, entering a weight in kg while the FMC is in lbs.

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Set your minima to those that would be given to you by ATC on approach. Mosts SIDS will have a maximum speed in the region of 220-230kts etc. If you can't find a published speed fly at no more than 230kts for the last segment of your SID. Descending from your SID to establish on the localiser 210kts. Once at FAP/FAF altitude slow down to 180kts. About 1-2DME from capturing the glideslope slow down to 160kts. About 6DME from the runway slow down to Vref+5. All jet transports are capable of these speed restrictions except Concorde. If you are a heavy and at maximum landing weight add 10kts.

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According to my minimum manouvre speeds chart for the 747, the lowest  Flaps 25 speed is 125 KIAS for a landing weight of 400,000 lbs. That would only be achievable with zero passengers/cargo and a negligable 7,000 lbs of fuel.

 

A more typical landing weight of say 500,000 lbs has a Flap25 minimum of 141 KIAS

 

You are flying too slow. What is your landing VREF speed in the FMC telling you ?

 

Should be no need to fly slower than 160 until about 3-5 miles from the threshold when you can reduce speed to VREF +

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As Peter says, 120KIAS is extremely low for even an empty B744 -- typical approach speeds with Flap 25 at typical landing weights would be more like 150-160KIAS.

 

In theory (i.e. in the real thing) the red bricks/yellow minimum manoeuvre margin indicators are driven by the stall warning computer (AoA vanes/IAS/etc etc) rather than by the FMC-entered gross weight, but this may not be true for the PMDG 747, I'm not sure.

 

It does sound as though you may have entered some weights incorrectly in general though if Vref25 is being computed as 115-120 KIAS. Are you sure you entered the ZFW in to the correct box (i.e. not in to the GW field) during the cockpit prep?

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Thanks for your replies. Sorry for the delay.

 

I usually just do things the lazy way and have the CDU "calculate" the zero-fuel weight by clicking the button twice. When I was testing things out to see what I might have done wrong, I didn't even program anything into the CDU, but the speed tape still showed the red bar at around 120 knots.

 

In retrospect, you're right that 120 knots is a very slow speed for any airliner. Even regional jets with full flaps aren't at 120 knots. At the time I figured the 747 had such different flight dynamics compared to any other airliner a 120 knot approach might have been feasible.

 

I should also note that in the PMDG 747, I've never seen the yellow bar at the minimum section of the speed tape. It's always been there for the max speed, but never the minimum. I'm not sure if that's due to my having an old version of the PMDG or my doing something wrong in the CDU.

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