Stabilized Approaches, Vref & Energy Levels

[Don Hudson]

I would like to ask a question regarding stabilized approaches. I know this is a bag of worms with lots of answers but I am considering just two of many factors in what we might call a Stabilized Approach - those two factors are Airspeed, and Groundspeed, as they relate to Vref. More specifically, they relate to some FOQA/FDA/FDM events I've been familiar with for a long time and I would like impressions and thoughts on these events and the actual circumstances they monitor.

The question actually came about while designing some FDM Events. The idea was to capture when either the approach speed, or the aircraft's energy level, was either too high or too low.

We are permitted to add as much as 20kts to Vref, (or 15kts to Vref +5!) but no more. We are not permitted to go below Vref.

The parameters used are "Calibrated Airspeed", "Vref", (actually, Vref +5kts because for many ops that's SOP, but not the wind/gust addition), ("Vapp" for Airbus) and "Ground Speed".

The Events are something like, "High (Low) Approach Airspeed Below nnnn ft AGL", which means a certain number of knots above Vref + 5, and below just "Vref", and "High/Low Energy on Approach Below nnnn ft AGL".

The idea is, if the ground speed is equal (roughly) to Vref to Vref + 5 then no matter what the airspeed is, (within limits of course!) the approach is, at that point, stable. But what if the groundspeed isn't equal to Vref/Vref +5? How to assess?

We know the problem of when the airspeed is beyond a stabilized limit and when it may not be.

A high airspeed and a normal groundspeed, (meaning airspeed 20kts above Vref but groundspeed at/near Vref) would be a normal response.

A low airspeed and a high groundspeed means a tailwind and depending upon how such wind shears out it could balloon the airplane or run out over a reasonable altitude, say 500ft.

Those flying the Airbus series with Groundspeed-Mini have a superb tool that is very good at bleeding/adding energy where needed, but especially with manual thrust, one still needs to remain aware of the aircraf energy level.

There are nine variations that can be put in a table, (obviously its infinitely variable, but for categorizing, nine airspeed/energy states work).

The table I built for thinking about this is below and I invite your comments on how this represents your experience and what you might expect as an FDM event monitoring trends in approaches below 1500ft AGL and especially at & below 500ft.

More to the point, do they represent reasonable descriptions/assessments of what is actually happening to the airplane and why, and do the categories provide sufficient guidance on the stability of the approach with reference just to these two circumstances?

Many thanks for any and all input!

Don

[inchman]

The one area where I would disagree with the Unstable tag is with Airspeed equal to Vref to Vref+20 with groundspeed below Vref in strong winds. The max allowable Vref increment is 20 kts, yet aircraft routinely and safely make approaches in winds greater than 30 kts (say.. 30G35). In this case, your table would show an unstable approach, which is not necessarily be the case.

One area of instability on approach that is overlooked is the tendency for some pilots to think of the Target airspeed as the minimum airspeed and some make quite drastic power changes in gusty conditions to avoid going below Target without regard for engine lag or kinetic energy.

In a lull in the wind, the airspeed drops momentarily, a mittfull of power is applied and, because of the nature of large engines, by the time they make the power actually comes up, the lull has passed the airspeed is back, yet the engine is now at high power and back on Target, but it takes some time for the engine to stop developing power when throttles are retarded...

... so now they're at Vref+30 with no apparent reduction coming (because the engines haven't de-spooled yet) so power is reduced further, and the aircraft is now back down to Target with low power just as the next lull arrives, so they end up at Vref+10, power at a minimimum... big jag of power...

So, IMO, an unstable approach, especially in a heavy aircraft, is indicated more by large power changes trying to chase Target than by fluctuations in airspeed or momentarily being below Vref(ground). I would rather be at Vref-1 with stable power and attitude than Vref+1 chasing every knot of speed change with throttle movement.

[Don Hudson]

Very helpful, inchman, thank you, especially your comment about the target speed being the "minimum" when inertia is the key, not being on-the-money 100% of the time with the target speed. FDM tends to be "snap-shotish" and we get around that by looking at a phase for a period of time and then either looking at the peak values over that period or a mean, or standard deviation, depending. I had thought of power levels but hadn't thought of variations in power as indications of a non-stable approach so I'm going to look at it and possibly work it in.

Cheers,

Don

[rudder]

Very helpful, inchman, thank you, especially your comment about the target speed being the "minimum" when inertia is the key, not being on-the-money 100% of the time with the target speed. FDM tends to be "snap-shotish" and we get around that by looking at a phase for a period of time and then either looking at the peak values over that period or a mean, or standard deviation, depending. I had thought of power levels but hadn't thought of variations in power as indications of a non-stable approach so I'm going to look at it and possibly work it in.

Cheers,

Don

Don - I know that some airlines include 'engines spooled' as a criteria for the 'stable' call at 1000'/500' (as applicable). I would tend to agree. If target EPR were approximately 1.18 I would not want to see 1.08 at 500' even if it was transitional.

[Don Hudson]

Don - I know that some airlines include 'engines spooled' as a criteria for the 'stable' call at 1000'/500' (as applicable). I would tend to agree. If target EPR were approximately 1.18 I would not want to see 1.08 at 500' even if it was transitional.

You know I completely agree and it's a puzzle to me why no publication of the usual stabilized approach criteria mentions "engines spooled". The usual phrase is, "power appropriate for the condition" and I don't think that's good enough...I've seen a number of "shuttle landings" in flight data over the years and "idle thrust" is "appropriate"...go figure.

What is a real puzzle are the long landings...really long...like 3000 to 4500ft past the threshold. The excuse has always been the length of the runway or "the airplane has a big wing and tends to float"...

Don

[J.O.]

Hi Don;

Unfortunately I no longer have the documentation which laid out the criteria and parameters we were using for an unstable approach in the Aerobytes program. For what it's worth, some SOPs are including the term "thrust above idle" in their stabilized approach criteria. I believe this is included in the standard FCOM from Airbus.

Jeff

[Moon The Loon]

Hi Don;

Unfortunately I no longer have the documentation which laid out the criteria and parameters we were using for an unstable approach in the Aerobytes program. For what it's worth, some SOPs are including the term "thrust above idle" in their stabilized approach criteria. I believe this is included in the standard FCOM from Airbus.

Jeff

But surely not in the flare? A peeve I always had with many European and other foreign pilots I flew with was their insistence on leaving "power above idle" for the touchdown. Invariably, the float lasted longer, and the landing distance was increased substantially.

There's no question though that, on final approach (gear and flap in landing position, and stabilized at final approach speed), engines must be spooled. It is impossible to maintain a 3 degree slope with full flap with engines at idle. On any aircraft I've ever flown, jet or turboprop. Unless you've got a wicked tailwind in which case, you probably shouldn't even be there...

[J.O.]

But surely not in the flare? A peeve I always had with many European and other foreign pilots I flew with was their insistence on leaving "power above idle" for the touchdown. Invariably, the float lasted longer, and the landing distance was increased substantially.

Well I would have thought that was obvious, but I understand your point.

It gets worse in the Airbus, which will add thrust to maintain airspeed if the thrust levers aren't brought to idle in the flare. Early in my training, I was told that there was no reason to ever leave the thrust levers above idle once you're past 50' AGL, even in gusty winds. I flew it that way for 16 years and landed in some pretty gusty conditions and never once had a problem making a safe landing in the touchdown zone. It used to surprise me how many pilots wanted to carry thrust above idle much lower, like down to 10 ft just because it was windy. Long landings were always the result and as recent history has shown, that's often just as dangerous as an unstable approach.

[Don Hudson]

Well I would have thought that was obvious, but I understand your point.

It gets worse in the Airbus, which will add thrust to maintain airspeed if the thrust levers aren't brought to idle in the flare. Early in my training, I was told that there was no reason to ever leave the thrust levers above idle once you're past 50' AGL, even in gusty winds. I flew it that way for 16 years and landed in some pretty gusty conditions and never once had a problem making a safe landing in the touchdown zone. It used to surprise me how many pilots wanted to carry thrust above idle much lower, like down to 10 ft just because it was windy. Long landings were always the result and as recent history has shown, that's often just as dangerous as an unstable approach.

Exactly J.O. In fact in one FDM Program, what I called the "thrust bump" on the Airbus could be seen and associated with long landings...With the TL's in "CLB", the N1's would begin to increase at about 20' (around the first 'retard' and voila, added energy in the flare, then the float and long landing. It was a standard pattern because the TLs hadn't been pulled back to the IDLE detent. I've seen it in other types as well..and I agree with your other comment, the thrust generally tends to stay on longer than necessary and higher speed/energy than necessary generally tends to be carried. The black tire marks on all runways are a kind of "data program" themselves; they show that most landings are beyond the 1500' hash marks. What aren't seen are the outliers further than 3000' past the threshold. The fact that even highly unstable approaches usually result in a rescued landing and not an overrun or undershoot appears to beg the stable SOPs but as we all know, it is habit, not just one approach that keeps the risk low. The fuel savings which come from SOPs requiring delayed gear, delayed landing flap selection and idle reverse thrust are tiny compared to the heightened risk but these decisions (to alter the SOPs) likely do not originate with the operations people. The QANTAS B747 overrun at Bangkok was such a case (the ATSB cited reduced landing flap setting and perhaps idle reverse thrust...I'd have to take a look at the report again) even as there were arguments in the cockpit about going around or staying on the runway.

[Moon The Loon]

Well I would have thought that was obvious, but I understand your point.

It gets worse in the Airbus, which will add thrust to maintain airspeed if the thrust levers aren't brought to idle in the flare. Early in my training, I was told that there was no reason to ever leave the thrust levers above idle once you're past 50' AGL, even in gusty winds. I flew it that way for 16 years and landed in some pretty gusty conditions and never once had a problem making a safe landing in the touchdown zone. It used to surprise me how many pilots wanted to carry thrust above idle much lower, like down to 10 ft just because it was windy. Long landings were always the result and as recent history has shown, that's often just as dangerous as an unstable approach.

Not having been an Airbus driver, I guess my comment was directed at REAL airplane pilots

Great discussion, Don/Jeff!

[Don Hudson]

Not having been an Airbus driver, I guess my comment was directed at REAL airplane pilots

Great discussion, Don/Jeff!

You mean like that Boeing plastic thing?...

DH...as in ducking and hiding...

[J.O.]

I'm in the midst of a new type rating course and I was pleasantly surprised to see the sim instructors making a point of showing us our touchdown points on the plot screen, especially when we've landed a bit long. Now they ask us what our height was over the threshold and where we think we touched down after most landings (except in low vis conditions). It really helps to concentrate on maintaining a stable profile and a touchdown in the right area.