Mirror Glideslope

[Henning]

Maybe this has already been explained and I missed it but - how do you capture a glideslope from above? Wouldn't it be behind you?

In front and behind unless you've crossed the runway threshold. It's below you otherwise interceptable by descending at an angle greater than the slope.

 

[RotorAndWing]

Maybe this has already been explained and I missed it but - how do you capture a glideslope from above? Wouldn't it be behind you?

If approaching from above (Airbus FCOM)

GLIDE SLOPE INTERCEPTION FROM ABOVE
The following procedure should only be applied when established on the localizer. There are a
number of factors which might lead to a glide slope interception from above. In such a case, the
crew must react without delay to ensure the aircraft is configured for landing before 1000 ft AAL.
In order to get the best rate of descent when cleared by ATC and below the limiting speeds, the
crew should lower the landing gear and select CONF 2. Speedbrakes may also be used, noting
the considerations detailed in the sub-section "Deceleration and configuration change" earlier in
this chapter. The recommended target speed for this procedure is VFE 2 - 5 kt. When cleared to
intercept the glide slope, the crew should:
• Press the APPR pb on FCU and confirm G/S is armed.
• Select the FCU altitude above aircraft altitude to avoid unwanted ALT*.
• Select V/S 1 500 ft/min initially. V/S in excess of 2 000 ft/min will result in the speed increasing
towards VFE

It is vital to use V/S rather than OP DES to ensure that the A/THR is in speed mode rather than
IDLE mode. The rate of descent will be carefully monitored to avoid exceeding VFE . When
approaching the G/S, G/S*will engage. The crew will monitor the capture with raw data (pitch and
G/S deviation). The go-around altitude will be set on the FCU and speed reduced so as to be
configured for landing by 1000 ft.
In such a situation, taking into account the ground obstacles and if ATC permits, it may be
appropriate to carry out a 360 °turn before resuming the approach.

 

[Engine]

Really? Under Normal Law how does that work? Please explain.

As I understand it Normal Law provides protection for a normal flight envelope. In this case the "aircraft rotated through 26 degrees nose up." The speed decreased to 130 knots. I assume this was a high AOA. The normal law should have protect against a high AOA. Based on the report had the AP not been disconnected it would've continued through 26 degrees to 30 degrees if the normal law was in fact working correctly. I assume the airspeed would've decreased as well. The aircraft was ultimately put into an "approach to stall" situation by the autopilot.

Airbus FCL Soruce

I am aware of the fact that the airplane never stalled. I wrote "stall" instead of "approach to stall." Didn't know I would be executed for it.

My experience with the aircraft is limited to Level-D simulators only. I do not make the claim to be an Airbus expert.

 

[RotorAndWing]

As I understand it Normal Law provides protection for a normal flight envelope. In this case the "aircraft rotated through 26 degrees nose up." The speed decreased to 130 knots. I assume this was a high AOA. The normal law should have protect against a high AOA. Based on the report had the AP not been disconnected it would've continued through 26 degrees to 30 degrees if the normal law was in fact working correctly. I assume the airspeed would've decreased as well. The aircraft was ultimately put into an "approach to stall" situation by the autopilot.

From the Airbus 319/320 FCOM:

HIGH PITCH ATTITUDE PROTECTION
Excessive pitch attitudes, caused by upsets or inappropriate maneuvers, lead to hazardous
situations:
• Too high a nose-up ▸ Very rapid energy loss
• Too low a nose-down ▸ Very rapid energy gain
Furthermore, there is no emergency situation that requires flying at excessive attitudes. For these
reasons, pitch attitude protection limits pitch attitude to plus 30 °/minus 15 °.
Pitch attitude protection enhances high speed protection, high load factor protection, and high
AOA protection.

HIGH ANGLE-OF-ATTACK (AOA) PROTECTION
High AOA protection enables the PF to pull the sidestick full aft in dangerous situations, and thus
consistently achieve the best possible aircraft lift. This action on the sidestick is instinctive, and the
high AOA protection minimizes the risk of stalls or control loss.
High AOA protection is an aerodynamic protection:
• The PF will notice if the normal flight envelope is exceeded for any reason, because the
autopitch trim will stop, the aircraft will sink to maintain its current AOA (alpha PROT, strong
static stability), and a significant change in aircraft behavior will occur.
• If the PF then pulls the sidestick full aft, a maximum AOA (approximately corresponding to CL
Max) is commanded. In addition, the speedbrakes will automatically retract, if extended.

In addition to this aerodynamic protection, there are three more energy features:
• If ATHR is in SPEED mode, the speed cannot drop below VLS, even if the target speed is below
VLS
• An aural low-energy "SPEED SPEED SPEED" warning, warms the flight crew that the energy of
the aircraft is below a threshold under which they will have to increase thrust, in order to regain
a positive flight path angle through pitch control. It is available in CONF 2, CONF 3, and CONF
FULL.
The FAC computes the energy level with the following inputs:
‐ Aircraft configuration
‐ Horizontal deceleration rate
‐ Flight path angle
For example, if the aircraft decelerates at 1 kt/sec, and:
‐ The FPA is -3 °, the alert will trigger at approximately VLS -8,
‐ The FPA is -4 °, the alert will trigger at approximately VLS -2.
This alert draws the PF's attention to the SPEED scale, and indicates the need to adjust thrust.
It comes immediately before the ALPHA Floor.
• If the angle-of-attack still increases and reaches ALPHA Floor threshold, the A/THR triggers
TOGA thrust and engages (unless in some cases of one engine-out).
In case of an emergency situation, such as Windshear or CFIT, the PF is assisted in order to
optimize aircraft performance via the:
• A/THR: Adds thrust to maintain the speed above VLS
• Low energy warning "SPEED, SPEED, SPEED": Enhances PF awareness
• ALPHA FLOOR: Provides TOGA thrust
• HIGH AOA protection: Provides maximum aerodynamic lift
• Automatic speedbrake retraction: Minimizes drag.

I am aware of the fact that the airplane never stalled. I wrote "stall" instead of "approach to stall." Didn't know I would be executed for it.

No execution, just clarify the situation as it does change what will happen. :wink2:

 

[Engine]

My point remains. The approach to stall was caused by the autopilot. Despite the AP and AT being engaged the aircraft was put into an approach to stall.

I understand the pitch limit is +30/-15. I assume that the AP was going for maximum lift in this instance. But it just doesn't seem right to me for it to approach 130 knots and through 26 degrees. The excerpt from the FCOM says "the high AOA protection minimizes the risk of stalls or control loss." However in this case the aircraft was put into the risk of a stall with AP & AT engaged. At least that is how I understand it.

 

[RotorAndWing]

My point remains. The approach to stall was caused by the autopilot. Despite the AP and AT being engaged the aircraft was put into an approach to stall.

I understand the pitch limit is +30/-15. I assume that the AP was going for maximum lift in this instance. But it just doesn't seem right to me for it to approach 130 knots and through 26 degrees. The excerpt from the FCOM says "the high AOA protection minimizes the risk of stalls or control loss." However in this case the aircraft was put into the risk of a stall with AP & AT engaged. At least that is how I understand it.

The FMGS took into account the pitch, airspeed, altitude, velocity, etc. They weren't at control loss yet, had it gone further they would have gone into apha prot.

The FMGS is not there to second guess the pilot and it does have parameters depending upon which regime of flight the aircraft is in. The crew took over and recovered before the aircraft attempted to do so.

 

[spinfire]

I encountered a false glide slope like this during my instrument training at KMHT, runway 17. Eventually recognized it and broke off the approach. It was a good teaching moment. I'm sure many people make it through instrument training without learning about the mirror/false glide slope phenomenon.

 

[RotorAndWing]

I encountered a false glide slope like this during my instrument training at KMHT, runway 17. Eventually recognized it and broke off the approach. It was a good teaching moment. I'm sure many people make it through instrument training without learning about the mirror/false glide slope phenomenon.

A good reason to verify crossing altitudes before and during the approach.

 

[Clark1961]

The Airbus autopilots will intercept from above or below. Some airports you will routinely get intercepts from above. RNWY 24 Manila (RPLL) is one that does it routinely as well as a few others.

Doesn't change my point. Can you see that?

 

[weilke]

I stand corrected about the location, if not the cause of possibly reflection off the ground or side bands creating the false glide slope.

Edit: I've been referring to side bands- I mean lobes from the antenna other than the main beam we should follow down the glide slope.

There are different methods to create the glideslope, some (most) of them actively use the surface in front of the antenna to create the lobes. A 'end fire' or 'waveguide' antenna that doesn't use the surface to reflect the signal is apparently a lot more expensive to build.

 

[Henning]

A good reason to verify crossing altitudes before and during the approach.

Yep, that would seem to make a lot of sense. Is there an answer to my previous question? It looked like it was flying the GS article it intercepted as a steeper than normal GS and was closing in on the correct point as the distance above the slope was steadily decreasing. Had they not done anything and left it alone in CATIII configuration, would it have completed a successful Autoland sequence?

 

[RotorAndWing]

Yep, that would seem to make a lot of sense. Is there an answer to my previous question? It looked like it was flying the GS article it intercepted as a steeper than normal GS and was closing in on the correct point as the distance above the slope was steadily decreasing. Had they not done anything and left it alone in CATIII configuration, would it have completed a successful Autoland sequence?

Nope.

From the article

the aircraft crossed the runway threshold at 2700 feet MSL (2370 feet AGL), the pitch attitude reduces to 5 degrees nose down and the aircraft rapidly descended.

Crossing the thresh hold at 2370 feet there is no way to correct for that into a stable landing either by hand flying or using the autopilots.

 

[RotorAndWing]

The autopilot manual (Altimatic IIIc) for the 'kota makes it clear that glide slope interception must be from below because of the problem of multiple glideslopes. The autopilot will not accept an intercept from above. Too bad Airbus doesn't use similar logic. Too bad the crew training didn't include that glideslope intercept must be from below. I had that training and I was my CFII's first student.

Doesn't change my point. Can you see that?

No, because in real world operations it doesn't work like that. Airbus and Boeing both have procedures for intercepting the GS from above. At some airports it's not uncommon to have an intercept to the localizer and be above the GS.

Your autopilot can't do it because of it's technology, plain and simple. More advanced systems it's not an issue.

The key is situational awareness, had the crew briefed intercept altitudes and the PM backed up the PF with a FAF crossing call this would have not happened.

 

[JeffDG]

For those who weren't aware of the false GS:
Instrument Flying Handbook, http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-15B.pdf, Page 9-40

2. False courses. In addition to the desired course, GS
facilities inherently produce additional courses at
higher vertical angles. The angle of the lowest of
these false courses occurs at approximately 9°– 12°.
An aircraft flying the LOC/GS course at a constant
altitude would observe gyrations of both the GS needle
and GS warning flag as the aircraft passed through the
various false courses. Getting established on one of
these false courses results in either confusion (reversed
GS needle indications) or in the need for a very high
descent rate. However, if the approach is conducted
at the altitudes specified on the appropriate approach
chart, these false courses are not encountered.

 

[JeffDG]

No, because in real world operations it doesn't work like that. Airbus and Boeing both have procedures for intercepting the GS from above. At some airports it's not uncommon to have an intercept to the localizer and be above the GS.

Your autopilot can't do it because of it's technology, plain and simple. More advanced systems it's not an issue.

The key is situational awareness, had the crew briefed intercept altitudes and the PM backed up the PF with a FAF crossing call this would have not happened.

You'd think that an advanced autopilot might be programmed to reject a glideslope with a 9 degree plus slope, ~3x the standard descent angle of an ILS.

Concur on situational awareness, but more checks in the system is rarely a bad thing.

 

[Henning]

You'd think that an advanced autopilot might be programmed to reject a glideslope with a 9 degree plus slope, ~3x the standard descent angle of an ILS.

Concur on situational awareness, but more checks in the system is rarely a bad thing.

Not that it would make a large difference percentage wise, but more lines of code in a computer system is rarely a good thing.

 

[JeffDG]

Not that it would make a large difference percentage wise, but more lines of code in a computer system is rarely a good thing.

OK, there's an excellent point right there.

 

[dell30rb]

So, I'm assuming that these artifact glide slopes still come in to the antenna creating a steeper glide slope. Given that, would the CatIII system still have been able to execute a successful landing arresting the higher sink rate at the bottom?

Yep that's how it works. I would think a heavy aircraft like an airbus would crash if it was coming down at a 9 degree angle. I don't think that descent rate would be possible at Vref in a normal landing configuration anyway.

At least the pilots ...eventually... disconnected the autopilot and hand flew it out of there.

 

[Silvaire]

In front and behind unless you've crossed the runway threshold. It's below you otherwise interceptable by descending at an angle greater than the slope.

I knew that