I understand hail can be very damaging to an aircraft, in particular to the engines and windshield.
In the event that a windshield becomes cracked so that visibility is impossible, can pilots pop out the windows when at a lower enough altitude in order to see the runway?
In the EMB-145 the abnormal procedure for obscured windscreens is to secure items in the cockpit, open the side window and look through its opening.
There are more aircraft that share this procedure.
Here is the QRH procedure for "Impaired or cracked windshield" in the EMB-145:
Associated Windshield Heating .............................. OFF
IF only the outer layer (glass) is cracked, no action is
required
IF inner layer of glass is cracked, proceed as follows:
Cockpit Door ......................................... CLOSE
Maximum Altitude ....................... 10,000 FT., OR MEA,
WHICHEVER IS HIGHER
Pressurization Manual Controller ........ 1 O'CLOCK POSITION
Wait 15 Seconds
Pressurization Mode Selector ........................... MAN
Cabin Delta-P ........................... SET EQUAL TO 1 PSI
Note: Pressurization Manual Controller must be used to set
and maintain Cabin Delta-P at 1 psi while descending
Airspeed .................................... BELOW 250 KIAS
Smoke Goggles .......................................... DON
In case both windshields are impaired:
Cabin (below 10,000 ft) ....................... DEPRESSURIZE
Airspeed ................................. MAXIMUM 140 KIAS,
MINIMUM Vref
IF forward view through both wind screens is impossible, secure
loose objects in cockpit and proceed:
Note: Intercommunication will be impossible with window
removed.
Direct Vision Window ................................ REMOVE
Landing must be made looking through the direct vision
window
* * * *
They use instruments instead.
BA flight 9 had it's windscreen rendered somewhat opaque by volcanic ash
As Flight 9 approached Jakarta, the crew found it difficult to see anything through the windscreen, and made the approach almost entirely on instruments, despite reports of good visibility. The crew decided to fly the Instrument Landing System (ILS); however, the vertical guidance system was inoperative, so they were forced to fly with only the lateral guidance as the first officer monitored the airport's Distance Measuring Equipment (DME). He then called out how high they should be at each DME step along the final approach to the runway, creating a virtual glide slope for them to follow. It was, in Moody's words, "a bit like negotiating one's way up a badger's arse." Although the runway lights could be made out through a small strip of the windscreen, the landing lights on the aircraft seemed to be inoperable. After landing, the flight crew found it impossible to taxi, due to glare from apron floodlights which made the already sandblasted windscreen opaque.
From Wikipedia
Some airplanes, like the Pilatus PC-12 pictured below, have a captain's side window that can be opened during landing so that the pilot has a direct view of the runway. If the windscreen is damaged, blocked, or iced-over, the captain can fly on instruments to the airport, then land by opening and looking out the little window.
If the aircraft has what is known as auto land, the auto pilot can land the airplane, so the pilot has that as an option. However, you can approach the runway in what is known as a forward slip. Using a lot of rudder, the tail will move to one side, this moves the nose to the opposite side, allowing the pilot to see forward, looking out the side window. The pilot also simultaneously uses opposite aileron, to counteract the turn, using the rudder would initiate. Once over the runway, at about 50 feet, the pilot would straighten the plane out, and begin the landing flare. He can then use the view of the the runway edge line, out his side window, to keep the plane aligned with the runway. A radar altimeter gives precise reading of how many feet he has to go, as he descends to the pavement. So, the pilot knows where he is. These things are practiced by flight crews in simulators twice a year.
Pilots tend to use the view out of the side windows in their peripheral vision to judge height above the runway, so provided you could see the approach lights at some point to know you are on the centre line, its not as bad as it sounds.
There are numerous examples in history of aircraft landing with oiled up, or iced up windscreens. Also it might be possible in some situations/aircraft to fly a curved approach then use the side windows in the final seconds of flight.
My view is that the reason why the wind shield is opaque was probably a bigger risk to the aircraft than actually landing with it in that condition.
Commercial aircraft pilots are required to pass an "Instrument Flight Rules" (IFR) rating. This rating requires the pilot to flight their aircraft using just the instruments in the cockpit.
As to how will the pilot know where to land. Commercial and military aircraft possess multiple instruments to provide location information. This includes GPS based instruments. I believe that this still includes VOR/DME (VHF omnidirectional range/distance measurement equipment) too but I've been out of the industry for a couple of decades and these may no longer be included.
At commercial & military aircraft flight speeds, busting out your windscreen would be a terribly bad idea - probably fatal. It would certainly make it so you wouldn't be able to land the plane. A better choice for non-instrument rated pilot would be to have the tower talk you in.
Per this other SE question & answer, although a true zero-zero landing is actually possible and safe to perform (provided ILS equipment is installed in aircraft and airport), there are problems on the ground with rollout and maneuvering on the ground which for practical purposes eliminate it except under extreme circumstances.
