Air Crash Investigations - question about an episode.

[flying dog]

Aircrash investigation S19 E10 Runway Breakup.

 

Overview:

Plane 737.

Pilot flying IMC into airport. (HEAVY rain)

Lands short - crashes.

 

Findings:

- The pilot used to fly turbo props and was used to doing landings "aiming for the green lights" rather than using the PAPI, or aiming for the "piano keys".

- The plane was descending at 1100 FPM. Boeing's specified max is 900.

- If the Rate of Descent is greater than 900 (fpm) it is an unstable approach.

- "He was coming in steep and ALWAYS below the glideslope."

 

My "problems":

In the review it is said:

In vertical speed mode the crew must manually select their rate of descent.

Um, duh! That's why it is called Vertical speed.

 

On the "He was coming in steep and always under the glideslope" part:

If the plane was ALWAYS under the glideslope, they wouldn't have reached Top Of Descent. Why were they going down?

Ok, maybe to try and sneak in under the weather.

 

Given the plane has a given Ground speed - and yes I get that this can vary with wind speed changes - the "Top of descent" is a function of altitude and ground speed if you want a constant rate of descent.

 

So, why wouldn't you use VS if you have a given altitude and ground speed?

 

It wasn't clear the airport had ILS. But did have PAPI lights.

 

ILS is better I know because if the wind speed changes - and therefore the ground speed - the plane's position on the glide slope will change and the plane will act accordingly.

 

On the Rate of Descent.

Doesn't that depend on the airspeed too?

If you are doing 600 kts IAS and a ROD of >900 FPM, yes you have a problem. But if you have an IAS of 160 kts and a ROD of >900 FMP, why is that a problem?

All the plane sees is "air over its wings" - be it going up, down, or level.

 

 

So I am not exactly sure I get why the crew using Vertical Speed mode is/was a problem.

[Flying Binghi]

I think yer needs one of them 737 cattle truck drivers to answer this. Probably a link to the report wouldn’t go astray...?

 

 

 

 

.

[Guest Machtuk]

If you use VS mode below a selected ALT (which isn't uncommon say below 3000' ) the A/C will continue that VS till it hits the ground. The crew where not minding the 'shop'! VS is often used in a NPA such as a LOC App where computed verticle guidance is not available in the FMS. As for flying turbo props and aiming at the green lights or piano keys that's an odd statement and a non Std proceedure. PAPI's or any other visual slope guidance devices can be deceptive in heavy rain due light refraction, also they are normally set up to display 'on slope' to the pilots operating the largest A/C using that airfield. You are correct TOD is a function of ALT but that profile can and does change on the way down due winds, ATC restrictions and or a change in track miles or controlled airspace steps, decent is a very fluid environment at times. So called professional pilots can stuff it up, humans are good at that despite all the fancy techno stuff right there in front of them!

[KRviator]

It wasn't an overly complicated accident, from what I can find, it was Aires 8520 that you're referring to, and it is commonly known as a "Black hole approach" that gives the illusion you are much higher on approach than you actually are, and you increase the descent rate to compensate. IF they were on autopilot then use of VS in pitch to correct the "above GP" sensation would be a normal reaction as you can vary the rate with the automatics until you're happy. Unfortunately, that doesn't always mean you're on profile...

[flying dog]

(Weird, I'm not getting notifications of replies.)

 

Sorry I can't give a link to the episode.

 

I've got a paytv subscription. So I watched it there and can't find a "free" version to share.

[flying dog]

That looks like it. But 3.33 of a 45 minute show isn't really going to give you much.

[Yenn]

What are the green lights the pilot was aiming for.

It is a well known effect that in heavy rain yo will get the illusion of being higher on the approach than you really are.

[Guest Machtuk]

What are the green lights the pilot was aiming for.

It is a well known effect that in heavy rain yo will get the illusion of being higher on the approach than you really are.

 

Runway Threshold lights are green, denotes the beginning of the actual Rwy.

[Yenn]

Oh I thought it was the TVasis light filtering through the green grass because you were too low.

[Jabiru7252]

What are the green lights the pilot was aiming for.

It is a well known effect that in heavy rain yo will get the illusion of being higher on the approach than you really are.

 

Who else knew this 'well known' effect? Anybody know why?

[poteroo]

Who else knew this 'well known' effect? Anybody know why?

 

Any 'suspended' particles in the air will cause distortion of light. eg, think of water as equivalent to air, (both are fluids, so fluid mechanics applies). If you add soil to the water, it becomes muddy, and to all intents - opaque. Less mud = better visibility through the fluid.

 

For pilots, think of smoke, fog, rain as being 'suspended' in the air. Light is deflected off any suspended solids by a process known as refraction. The net effect of this is that you cannot see 'as far' through the air, and so - objects are actually closer to you than what they would be in clear air. Your eyes, of course, have been trained to accommodate to the clear air, (VMC/VFR), situation, and thus, the unwary pilot is closer to obstacles than 'thought'. A CFIT becomes more of a probability.

 

 

On approach, in less than VMC, and having a PAPI to help, it's probably wise to stay above the standard mid-way setting to allow for the visual illusion.

 

My cohort of PNG pilots were trained, (ICUS), to operate with lots of water in the air, (usually as cloud, mist or rain of varying intensity). This was all part & parcel of the old ANO 28.1 which spelled out the requirements for commercial pilot route & airstrip endorsement, without which a CPL could not work in PNG. It still exists today.

 

As well as learning all the landmarks of the valleys & gaps, and each airstrips quirks, we were trained to operate our steeds at KIAS lower than Va, and how to make minimum radius turns in confined amounts of airspace with visibilities of well below the minima. This usually meant 20o flap and plenty of power to keep from Vso in a 45o A-O-B turn. We learned a lot about staying visual!

 

Much of this training carried through when I did my ag rating with Max Hazelton at Cudal in early 1971. Knowing just where to position your aircraft, and at what configuration and speed, goes a long way to staying safe at low altitudes. Today, I still teach low level, (some 53 years since receiving it myself), and I try to expose my trainees to some reduced visibility flight, in which to practise their handling skills.

 

IMHO, this flying skill should be taught to every pilot as part of the necessary syllabus because not everyone has a full panel of attitude instruments, and even less pilots are competent to transition to non VMC flight. But, that's another story, as the regulators continue in denial of the accident rate in the lower levels.

 

Hope this answers the question? Happy days,

[Jabiru7252]

A good part of my work requires an understanding of atmospheric transmission of light and infrared radiation through the atmosphere. I can understand that one cannot see as far is dirty air as they can in clear air but I do not understand how/why that makes an object appear closer. Certainly haven't come across that in my studies. However, my brain has started to get lazy so I don't push it hard as much as I'd like to.....

[facthunter]

Refraction in rain causes any VASI to be considered unreliable. Heavy showers often include downdrafts as well. Keep a bit higher than you would normally on approach in these circumstances. Nev

[Yenn]

Jab 7252.

The well known effect is known to most people who take a learning interest in things aviation. CASA has talked about it and it has come up at their safety seminars. I have seen it mentioned in several aviation how to books. I consider it is so well known, that if you didn't know it, you are not actively trying to educate yourself in aviation matters.

The other thing I mentioned was green being because you are low and the light is filtered by the grass. That comes from a talk given to a group of bankers by a pilot, who also told them that if they were still in the plane after he had told them to evacuate, then they should assume they were pilot in command. A very funny and educating recording, which I seem to have lost. Bugger!

[facthunter]

The VASI/ PAPI lights themselves are well above the surface of the ground.( about Knee height) and would be showing ALL RED long before you are looking through the grass and it's based on a 3 degree slope. I've never heard of THAT turning green effect and I cannot see how it could happen at all. Nev

[Yenn]

It was part of a hilarious talk. Plenty of other things like that.